Contribution of Gestational Age and Birth Weight to Perinatal Viability in Singletons Versus Twins

The objective of this study was to determine the effects of birth weight and gestational age on twin vs. singleton mortality. Population-based analysis of live births, fetal deaths, and infant deaths by plurality in the United States from 1983 to 1986 was conducted. Seven mortality rates and relative risks (RRs) of twin vs. singleton mortality were calculated by birth weight, gestational age, and combined birth weight and gestational age. The mortality rates included fetal, perinatal, early neonatal, late neonatal, neonatal, postneonatal, and infant. Twins had 3–4 times the RRs of mortality compared to singletons, ranging from a RR of 2.71 for postneonatal mortality to a RR of 3.73 for late neonatal mortality. Generally, for birth weights of 2,800 g or less and gestational ages of 38 weeks or less, twins had lower combined birth weight and gestational age mortality rates and lower RRs. Between 1,900 and 2,799 g, mortality rates decreased then increased with advancing gestation between 31 and 42 weeks both more severely and consistently for twins than for singletons. In conclusion, twins have lower birth weight and gestational age-specific mortality rates and RRs than singletons below 2,800 g and 39 weeks. The “U”-shaped pattern of mortality beyond 38 weeks gestation, particularly for twins with birth weights below 2,500 g, reflects the combined influence of growth retardation and advancing gestation on mortality. The lowest mortalityfor twins is achieved at birth weights of 2,500-2,799gat35-38 weeks gestation. Only 1 in 7 twins is born within this “ideal window.” Efforts at reducing twin mortality should be directed toward reducing intrauterine growth retardation and achieving optimal timing for delivery.     

Customized birth weight for gestational age standards: Perinatal mortality patterns are consistent with separate standards for males and females but not for blacks and whites

BACKGROUND: Some currently available birth weight for gestational age standards are customized but others are not. We carried out a study to provide empirical justification for customizing such standards by sex and for whites and blacks in the United States. METHODS: We studied all male and female singleton live births and stillbirths (22 or more weeks of gestation; 500 g birth weight or over) in the United States in 1997 and 1998. White and black singleton live births and stillbirths were also examined. Qualitative congruence between gestational age-specific growth restriction and perinatal mortality rates was used as the criterion for identifying the preferred standard. RESULTS: The fetuses at risk approach showed that males had higher perinatal mortality rates at all gestational ages compared with females. Gestational age-specific growth restriction rates based on a sex-specific standard were qualitatively consistent with gestational age-specific perinatal mortality rates among males and females. However, growth restriction patterns among males and females based on a unisex standard could not be reconciled with perinatal mortality patterns. Use of a single standard for whites and blacks resulted in gestational age-specific growth restriction rates that were qualitatively congruent with patterns of perinatal mortality, while use of separate race-specific standards led to growth restriction patterns that were incompatible with patterns of perinatal mortality. CONCLUSION: Qualitative congruence between growth restriction and perinatal mortality patterns provides an outcome-based justification for sex-specific birth weight for gestational age standards but not for the available race-specific standards for blacks and whites in the United States.     

Fetal growth and perinatal mortality in twin pregnancy - effect of sick leave and hospitalization

Fetal growth, birth weight specific mortality rates and effect of sick leave or hospitalization on the fetal growth were investigated in a material of 476 twin pregnancies managed at University Central Hospital of Turku in year 1970–1981. Birth weights of twin babies at any gestational age were slightly but not significantly higher than in earlier materials. When compared to growth curve of singleton fetuses, the growth rate of both twins is equal to singletons up to 30th week of pregnancy, being thereafter slower than in singleton pregnancies.Although duration of sick leave and hospitalization increased considerably during the study period, no change in the duration of pregnancy nor in the weight of twin babies occurred. Instead perinatal mortality decreased from 101/per thousand to 36.2/ per thousand. Birth weight specific mortality rates did not differ from those in singleton fetuses.     

Fetal and neonatal mortality risks of multiple births

The purpose of this article is to describe the perinatal mortality experience and mortality-related risk factors of recent US multiple births. First, we describe trends in fetal and neonatal mortality rates for singleton and multiple births to understand if the improvements in perinatal mortality in the United States are equally or differentially reflected among multiple births. Because the characteristics of women who have multiple deliveries differ from those of mothers of singletons, we describe the risk of fetal and neonatal mortality by maternal characteristics and plurality. Finally, we examine the distribution and fetal and neonatal mortality risk of singleton and multiple births by birth weight and gestational age to provide an updated assessment and contrast of their comparative survival chances within similar birth weight-gestational age categories of intrauterine development.     

Effect of prolongation of pregnancy on perinatal mortality

Objectives: (1) To determine the distribution of singleton and twin births according to gestational age in a Nigerian obstetric population; and (2) to compare their perinatal outcomes according to gestational age. Methods: A 10-year retrospective comparative study of twin and singleton births at a tertiary care center in Enugu, Nigeria. The variables analyzed were: the proportion of deliveries occurring at each gestational age, the gestational age-specific rates for stillbirths, cesarean section, babies with 1-min Apgar scores less than 4 and those whose birthweights were below the 10th percentile for gestational age. The trends in these rates were determined by finding the best fitting polynomial regression curve for each variable. Tests of statistical significance for trends in proportions were carried out by means of the χ²-test at the 95% confidence level. Results: Of the 496 twin births, 3.6% compared with 17.3% of the 496 singleton births went beyond 40 weeks’ gestation while 1.2% of the twin and 4.4% of the singleton deliveries occurred at 42 weeks’ gestation or beyond. For twins as well as singletons, there was a consistent and significant decline in the stillbirth rate and the proportion of babies with 1-min Apgar scores less than 4 up to 42 weeks (P=0.0000). Among the twins, the proportion of babies with birthweights below the 10th percentile (i.e. those with impaired growth) significantly rose from 28 weeks and above (P=0.0000) while among the singletons, a declining trend with gestational age was observed (P=0.0003). However, among the twins with impaired growth, the stillbirth rate neither differed between the first and second twins at each gestational age nor did it increase with gestational age in both the first and second twins. While the cesarean section rate for singletons remained almost stable at approximately 13%, there was a significant rise in the cesarean section rate with gestational age among the twin births. Conclusions: There were 1.2% of twin deliveries compared with 4.4% of singleton deliveries which occurred at 42 weeks’ gestation or beyond. In the Nigerian population studied, the perinatal outcomes in twins did not differ from those of singletons up to 42 weeks’ gestation suggesting that the 42-week cut-off for prolonged pregnancy applies equally well to twins as to singletons.     

Perinatal mortality and morbidity rates among singleton,twin, and triplet gestations

Objective: To determine rates of perinatal mortality and morbidity from 24 to 43 weeks gestation among singletons, twins, and triplets.Methods: Successfully linked data from 1992 Californian maternal and infant discharge records as well as birth and death certificates from acute care civilian hospitals were examined for perinatal mortality and morbidity. Perinatal mortality was defined as the sum of all stillbirths and neonatal deaths. Deliveries from 24 to 43 weeks gestation among singleton, twin, and triplet pregnancies were collected as separate data sets. Perinatal mortality was identified using birth certificate death indicators excluding deaths caused by congenital anomalies. Neonatal deaths were identified from death indicators found in the death certificates. For the purpose of this study, perinatal morbidities were identified by ICD-9 codes and limited to respiratory distress syndrome (RDS), intraventricular hemorrhage (IVH), and necrotizing enterocolitis (NEC). Perinatal mortality and morbidity rates were expressed as a percent of live births stratified by gestational age. Perinatal mortality data were expressed in log scale and perinatal morbidity rates were statistically compared.Results: There were 571,390 total births in California of which 527,677 (92%) were singleton, 12,535 (2%) were twin, and 367 (0.06%) were triplet gestations. Across gestation, the rate of RDS between triplets and twins was comparable (6.6% vs 6.8%). However, the rates of IVH and NEC were significantly greater in triplets than in twins (20% vs 8%, P < .0001, and 25% vs 9%, P < .0001, respectively). The perinatal mortality rates are shown below.

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Conclusions: Perinatal mortality rates were comparable among singleton, twin, and triplet gestations delivered between 24 and 30 weeks gestation. Unlike singletons and twins, the triplet perinatal mortality rate did not fall between 31 and 36 weeks gestation and remained at 2.6%. Twin perinatal mortality rate was equivalent to singletons until 36 weeks gestation. IVH and NEC were significantly greater among triplets regardless of gestational age. These data suggest that antepartum fetal surveillance of triplet pregnancies should start as early as 30 weeks gestation while testing for twin pregnancies can begin at 36 weeks gestation.     

The changing epidemiology of multiple births in the United States

OBJECTIVE: To describe changes in the epidemiology of multiple births in the United States from 1980 to 1999 by race, maternal age, and region; and to examine the impact of these changes on birth weight-specific infant mortality rates for singleton and multiple births. METHODS: Retrospective univariate and multivariable analyses were conducted using vital statistics data from the National Center for Health Statistics. RESULTS: Between 1980 and 1999, the overall multiple birth ratio increased 59% (from 19.3 to 30.7 multiple births per 1000 live births, P <.001), with rates among whites increasing more rapidly than among blacks. Women of advanced maternal age, especially those aged 30-34, 35-39, and 40-44 experienced the greatest increases (62%, 81%, and 110%, respectively). Although all regions of the United States experienced increases in multiple birth ratios between 1991 and 1999, the Northeast had the highest twin (33.9 per 1000 live births) and higher order birth ratios (280.5 per 100,000 live births), even after adjusting for maternal age and race. Between 1989 and 1999, multiple births experienced greater declines in infant mortality than singletons in all birth weight categories. Consequently, very low birth weight and moderately low birth weight infant mortality rates among multiples were lower than among singletons. CONCLUSION: It is important to understand the changing epidemiology of multiple births, especially for women at highest risk (advanced maternal age, white race, Northeast residents). The attribution of infertility management requires further study. The differential birth weight-specific infant mortality for singletons and multiples demonstrates the importance of stratifying by plurality when assessing perinatal outcomes.     

Perinatal outcome and peripartum complications in preterm singleton and twins deliveries: a comparative study

OBJECTIVE: Multiple pregnancy is one of the major risk factors for preterm births. The aim of the present study was to compare perinatal outcome and peripartum complications between twins and singletons, born preterm. STUDY DESIGN: The study population consisted of preterm deliveries of 435 pairs of twins (870 neonates) and the comparison group included 4754 preterm deliveries of singletons, born in the same period (January 1, 1989-December 31, 1996). Exclusion criteria were lack of prenatal care and births following infertility treatments. The three steps in statistical analysis consisted of (1) degree of concordance between the twins; (2) comparison between each twin (I and II) to their singleton comparison groups using SPSS computer program; (3) stratified analysis to examine perinatal mortality rates at different gestational age groups. RESULTS: The prevalence of preterm deliveries was 7.9% (6192/77610). Perinatal mortality was lower in twins of both birth orders, however, it was statistically significant only when APD is considered. Mortality rates in all gestational age groups and for both twin groups were lower than that of singleton [OR=0.45 (0.26-0.75; 95% CI) for twin-I; OR=0.36 (0.21-0.59; 95% CI) for twin-II]. Compared to singletons, twin gestations had less congenital malformations. Twin gestation had statistically lower rates of preterm premature rupture of membranes, severe pregnancy induced hypertension, oligohydramnios, placenta previa, placental abruption and clinical chorioamnionitis [12.2 vs.17.3%, 2.5 vs. 6.3%, 2.3 vs. 4.7%, 0.9 vs. 2.9%, 1.8 vs. 5%, 1.8 vs. 5.2%, respectively (P<0.01)]. Mothers of twins had less diabetes mellitus class B-R, hydramnios and chronic hypertension than that of singleton (1.8 vs. 2.6%, 5.5 vs. 7.4%, 3.7 vs. 4.8%, respectively). Cesarean section rates were significantly higher in twin's gestation. Mothers of twins tended to be older and of higher birth and gravidity order. CONCLUSIONS: Perinatal mortality rates and peripartum complications were lower in twin compared to singleton gestations.     

Theory of obstetrics: An epidemiologic framework for justifying medically indicated early delivery

Background

Modern obstetrics is faced with a serious paradox. Obstetric practice is becoming increasingly interventionist based on empirical evidence but without a theoretical basis for such intervention. Whereas obstetric models of perinatal death show that mortality declines exponentially with increasing gestational duration, temporal increases in medically indicated labour induction and cesarean delivery have resulted in rising rates of preterm birth and declining rates of postterm birth. Other problems include a disconnection between patterns of gestational age-specific growth restriction (constant across gestation) and gestational age-specific perinatal mortality (exponential decline with increasing duration) and the paradox of intersecting perinatal mortality curves (low birth weight infants of smokers have lower neonatal mortality rates than the low birth weight infants of non-smokers).

Discussion

The fetuses at risk approach is a causal model that brings coherence to the various perinatal phenomena. Under this formulation, pregnancy complications (such as preeclampsia), labour induction/cesarean delivery, birth, revealed small-for-gestational age and death show coherent patterns of incidence. The fetuses at risk formulation also provides a theoretical justification for medically indicated early delivery, the cornerstone of modern obstetrics. It permits a conceptualization of the number needed to treat (e.g., as low as 2 for emergency cesarean delivery in preventing perinatal death given placental abruption and fetal bradycardia) and a calculation of the marginal number needed to treat (i.e., the number of additional medically indicated labour inductions/cesarean deliveries required to prevent one perinatal death). Data from the United States showed that between 1995–96 and 1999–2000 rates of labour induction/cesarean delivery increased by 45.1 per 1,000 and perinatal mortality decreased by 0.31 per 1,000 total births among singleton pregnancies at > = 28 weeks of gestation. The marginal number needed to treat was 145 (45.1/0.31), showing that 145 excess labour inductions/cesarean deliveries in 1999–2000 (relative to 1995–96) were responsible for preventing 1 perinatal death among singleton pregnancies at > = 28 weeks gestation.

Summary

The fetuses at risk approach, with its focus on incidence measures, provides a coherent view of perinatal phenomena. It also provides a theoretical justification for medically indicated early delivery and reconciles the contemporary divide between obstetric theory and obstetric practice.     

Infants' length at birth: an independent effect on perinatal mortality

AIM: To investigate whether variations in birth length (crown-heel-length) were associated with perinatal mortality rate independent of birth weight. MATERIAL: The study population was singleton live- and stillbirths from 16 weeks of gestation compiled in the Medical Birth Registry of Norway from 1967 to 1997, totaling 1,705,652 births. METHOD: The total population was analyzed using z-scores for length at birth, birth weight and gestational age. Variation in perinatal mortality by length at birth was studied within birth weight strata (250 g) by logistic regression. RESULTS: Perinatal mortality varied more by birth length than by birth weight or gestational age, especially for values above the population means. Within birth weight strata, the association between perinatal mortality and length was similar in all 250 g birth weight categories above 1,500 grams: mortality was lowest at birth lengths 0-2 cm below average, with mortality rates increasing exponentially in either direction. CONCLUSION: Within all birth weight strata, and adjusted for gestational age, long infants had the higher risk of perinatal death, suggesting that length at birth may be a valuable predictor when assessing the risk of perinatal mortality.     

Perinatal outcome of ICSI pregnancies compared with a matched group of natural conception pregnancies in Flanders (Belgium): a cohort study

A retrospective cohort study was conducted with an intracytoplasmic sperm injection (ICSI) group and a naturally conceived comparison group. A total of 1655 singleton and 1102 twin ICSI births were studied with regard to perinatal outcome. Control subjects (naturally conceived pregnancies) were selected from a regional registry and were matched for maternal age, parity, place of delivery, year of birth and fetal sex. The main outcome measures were duration of pregnancy, birth weight, Apgar score <5 after 5 min, neonatal complications, perinatal death and congenital malformations. Twin births, when compared with singletons, carry a much higher risk of poor perinatal outcome. For both ICSI singletons and ICSI twins, no significant difference was found between ICSI and naturally conceived pregnancies for all investigated parameters. After excluding like-sex twin pairs, ICSI twin pregnancies were at increased risk for perinatal mortality (OR = 2.74, CI = 1.26-5.98), prematurity (OR = 1.38, CI = 1.10-1.75) and low birth weight (OR = 1.34, CI = 1.06-1.69) compared with spontaneously conceived different-sex twin pairs. In conclusion, the perinatal outcome of ICSI singleton and twin pregnancies was very similar to that of spontaneously conceived pregnancies in this large cohort study. After excluding like-sex twin pairs, ICSI twins were at increased risk for prematurity, low birth weight and higher perinatal mortality compared with the natural conception comparison group.     

Physical examination-indicated cerclage in singleton and twin pregnancies: maternal–fetal outcomes

Objectives: To study maternal and perinatal outcomes after physical examination-indicated cerclage in both singleton and twin pregnancies and evaluate the possible risk factors associated.

Study design: Retrospective review of all women undergoing physical examination-indicated cerclage at the Hospital Vall d’Hebro, Barcelona from January 2009 to December 2012 after being diagnosed with cervical incompetence and risk of premature birth.

Results: During the study period, 60 cases of women diagnosed with cervical incompetence who were carrying live and morphologically-normal fetuses (53 singleton and 7 twin pregnancies), and who had an imminent risk of premature birth were evaluated. Mean gestational age until birth was 35 weeks in singleton and 32 weeks in twin pregnancies. Four cases (7.5%) of immature births and one case (2.0%) of neonatal death were recorded in singleton pregnancies. No cases of immature births or neonatal deaths were recorded in twin pregnancies. Diagnostic amniocentesis was performed IN all cases to rule out possible chorioamnionitis.

Conclusions: Physical examination-indicated cerclage for cervical incompetence in women at risk for immature or preterm birth demonstrates good perinatal prognosis without increasing maternal morbidity in either singleton or twin pregnancies. The increase in gestation time in our study may also have been due to the fact that patients with subclinical chorioamnionitis were excluded by diagnostic amniocentesis.     

Prospective risk of fetal death in singleton, twin, and triplet gestations: implications for practice

OBJECTIVE: To evaluate the prospective risk of fetal death in singleton, twin, and triplet pregnancies and to compare this risk with fetal and neonatal death rates. METHODS: We analyzed 11,061,599 singleton, 297,622 twin, and 15,375 triplet gestations drawn from the 1995-1998 National Center for Health Statistics linked birth and death files. Prospective risk of fetal death was expressed as a proportion of all fetuses still at risk at a given gestational age and compared with fetal death rate. Fetal death risk and neonatal death rates were represented graphically for singletons, twins, and triplets. RESULTS: The prospective risk of fetal death at 24 weeks was 0.28 per 1000, 0.92 per 1000, and 1.30 per 1000 for singletons, twins, and triplets, respectively. At 40 weeks, the corresponding risk was 0.57 per 1000 and 3.09 per 1000 for singletons and twins, respectively and, at 38 or more weeks, 13.18 per 1000 for triplets. Plots of gestation-specific prospective risk of fetal death and neonatal mortality converged for singletons and twins at term but crossed for triplets at approximately 36 weeks' gestation. CONCLUSION: Prospective risk of fetal death is greater for triplets and twins than for singletons and greater for triplets than for twins during the third trimester. The pattern corroborates with uteroplacental insufficiency as a suspected underlying mechanism. When prospective risk of fetal death exceeds neonatal mortality risk, delivery might be indicated. When this model is used, this data set suggests that it might be reasonable to consider delivery of twins by 39 weeks and triplets by 36 weeks to improve perinatal outcome.     

Medical and economic effects of twin gestations

OBJECTIVE: To determine the incidence and trends of twinning in the United States and to review the medical and economic effects of twin versus singleton gestations. METHODS: Pertinent and recent studies on twin gestations were obtained through a MEDLINE database search of the English language between December 1987 and December 1999. Data from the 1995-1996 National Center for Health Statistics were also used to compare gestational age at delivery, fetal growth restriction, and perinatal mortality for twin and singleton gestations. Studies that have evaluated perinatal risks in relation to advanced reproductive technology also were reviewed and summarized. The economic implications of twinning from a societal perspective and infant quality of life issues of twins compared with singleton gestations are reviewed. RESULTS: Due to delayed childbearing and increased use of reproductive technologies, the incidence of twin gestations in the United States has been increasing. Twin pregnancies have a higher risk of complications, including pregnancy-induced hypertension, anemia, antepartum and postpartum hemorrhage, and maternal mortality. In addition, twin infants are more likely to deliver preterm, have low birth weight and greater perinatal mortality rates. These outcomes influence health care costs and quality of life for both parents and children. CONCLUSIONS: Women carrying twin fetuses are at increased risk for perinatal and obstetric complications. The increased perinatal risks that accompany twin fetuses may be partly due to the increasing use of advanced reproductive technologies. The economic burdens, as well as the potential for decreased quality of life among twins, needs careful evaluation.     

Twins and triplets: the effect of plurality and growth on neonatal outcome compared with singleton infants

OBJECTIVE: Information on outcome by gestational age from large numbers of twins and triplets is limited and is important for counseling and decision-making in obstetric practice. We reviewed one of the largest available neonatal databases to describe mortality and morbidity rates and growth in newborn infants from multiple gestations and compared these data with data for singletons. STUDY DESIGN: Data from a large prospectively recorded neonatal database that incorporated neonatal records from January 1997 to July 2002 were reviewed. We evaluated birth weight and neonatal mortality and morbidity rates that affected long-term outcome for each week of gestational age from 23 to 35 weeks of gestation for all nonanomolous inborn twins and triplets who were admitted to the neonatal intensive care unit and compared these data to all singletons who met similar criteria during the same time period. RESULTS: There were 12,302 twin and 2155 triplet births that met the entry criteria. The data for these newborn infants were compared with 36,931 singletons. Average birth weights at each gestational week were similar for all gestational ages until 29 weeks of gestation for triplets and 32 weeks of gestation for twins. After these gestational ages, the entire difference between twins and singletons was due to the weight of the smaller twin; the larger twins' mean weights were similar to singletons at all weeks that were studied. Birth order at each week also did not affect neonatal mortality rates, even when corrected for route of delivery and antenatal steroids. Neonatal morbidities associated with adverse long-term outcomes (intraventricular hemorrhage, retinopathy of prematurity, necrotizing enterocolitis) were also not different between multiple infants and singletons. Intrauterine growth restriction (IUGR) was associated with increased mortality rates at all gestational ages, but in the absences of IUGR, discordance was not. CONCLUSION: Data on a large number of twins and triplets provide reassurance that neonatal outcome at all viable premature weeks of gestation are similar to singletons. Intrauterine growth restriction and prematurity are therefore the principal issues that drive neonatal mortality and morbidity rates in multiple gestations. These data are important for obstetric decision-making and patient counseling.     

Perinatal mortality in first- and second-born twins in the United States

OBJECTIVE: To evaluate the prevailing mortality paradox that second-born twins are at higher risk of perinatal mortality than first-born twins. METHODS: We used the 1995-1997 United States "matched multiple birth" data files assembled by the National Center for Health Statistics, for analysis of risk of perinatal mortality in first- and second-born twins (293788 fetuses). Perinatal mortality was defined to include stillbirths after 20 weeks of gestation and neonatal deaths (deaths within the first 28 days). Gestational age-specific risk of perinatal mortality (per 1000 total births), stillbirth (per 1000 total births), and neonatal mortality (per 1000 livebirths) by order of twin birth were based on the fetuses-at-risk approach. Associations between order of birth and mortality indices were evaluated by fitting multivariable logistic regression models based on the method of generalized estimating equations. These models were adjusted for several potential confounding factors. RESULTS: Perinatal mortality was 37% higher in second-born (26.1 per 1000 total births) than in first-born (20.3 per 1000 total births) twins (adjusted relative risk [RR] 1.37; 95% confidence interval [CI] 1.32, 1.42). The increased risk of perinatal mortality in second-born twins was chiefly driven by a 2.46-fold (95% CI 2.29, 2.63) increase in the number of stillbirths. However, the risk of neonatal mortality was very similar between first- and second-born twins (RR 0.99, 95% CI 0.95, 1.04). CONCLUSIONS: The increased risk of perinatal death in second-born twins is driven chiefly by increased rates of stillborn second twins. Thus, the increased mortality in second-born over first-born twins probably is an artifact of mortality comparisons.     

Gestational age-specific distribution of twin birth weight discordance

AIM: To examine the gestational age-specific distribution of twin birth weight discordance. METHODS: We analyzed all liveborn twin sets between 28 and 40 weeks' gestation from the United States 1995-1998 Multiple Matched Birth Data Set compiled by the National Center for Health Statistics. We calculated the 50th and 95th percentiles of birth weight discordance at each gestational age. Neonatal mortality rates were calculated for discordant twins at the 95th percentile of birth weight discordance for each gestational age. RESULTS: At older gestational ages, the 95th percentile of birth weight discordance resulted in an inter-twin birth weight difference of approximately 25%, a value often used to define twins as birth weight discordant. However, at earlier gestational ages, the 95th percentile of birth weight discordance was greater, reaching nearly 50% at 28 weeks. CONCLUSIONS: The inter-twin birth weight difference at the 95th percentile is greater at lower gestational ages, possibly illustrating the different nature or severity of twin birth weight discordance at an earlier gestational age.     

Double jeopardy: twin infant mortality in the United States, 1983 and 1984

The United States Linked Birth/Infant Death Data Sets: 1983 and 1984 Birth Cohorts from the National Center for Health Statistics were used to identify maternal and infant characteristics related to twin infant mortality; 41,554 white and 10,062 black live-born matched twin pairs were evaluated. Twin birth weight distribution was skewed with 48% of white and 63% of black twins born weighing less than 2500 gm. Overall infant mortality rates were 47.1 and 79.3 deaths per 1000 live births for white and black twins, respectively (five times the rates for singletons). Three fourths of deaths were among twins weighing less than 1500 gm. White like-gender twins had about twice the risk of both twins dying compared with unlike-gender twins. Likewise, white twin pairs with greater than 25% birth weight disparity had a 40% to 80% increased risk of both twins dying, compared with twins whose weights were within 10% of each other. Twins born to high-risk women (on the basis of demographic factors) were twice as likely to die as twins born to low-risk women. Thus strategies to decrease twin infant mortality must address both maternal and infant risk factors.     

Preterm and term births of small for gestational age infants: a population-based study of risk factors among nulliparous women

Objective To study risk factors for small for gestational age (SGA) infants by gestational age among nulliparous women and to estimate mortality rates among SGA and appropriate-for-gestational-age (AGA) infants by gestational age.
Design A population-based study from the Swedish Medical Birth Register.
Setting Sweden 1992–1993.
Population Liveborn singleton infants to nulliparous women (   n = 96,662  ).
Main outcome measures Crude and adjusted odds ratios of risk factors for SGA by gestational age. Rates of neonatal and postneonatal mortality.
Results Older maternal age (≥ 30 years) was foremost associated with increased risks of very and moderately preterm SGA (≥ 32 weeks and 33–36 weeks, respectively), but also with term SGA (≥ 37 weeks). Risks of SGA increased with decreasing maternal height at all gestational ages. Smoking increased the risks of moderately preterm and term SGA. Short maternal education increased the risk of preterm SGA and low pre-pregnancy body mass index slightly increased the risk of term SGA. Pre-eclampsia and essential hypertension foremost increased the risk of very preterm SGA (OR = 40.5 and 32.4, respectively) and moderately preterm SGA (OR = 17.4 and 10.6, respectively), but also increased the risk of term SGA. Neonatal and postneonatal mortality rates of SGA infants were substantially influenced by gestational age, and mortality rates were consistently higher among preterm SGA infants compared with AGA infants.
Conclusions Risk factors for SGA and mortality rates among SGA infants vary by gestational age. A subdivision of risk factors by gestational age adds knowledge, particularly about risks of preterm SGA, where the highest rates of mortality were observed.     

Neurodevelopmental outcomes of twins

In population-based studies, the prevalence of neurodevelopmental disability is consistently higher in twins than singletons. This is largely because birth weight and gestational age (GA) distributions of twin births are shifted to the left when compared with singleton births, and lower birth weight and lower GA are associated with increased risk of neurodevelopmental disability. From a pathophysiologic perspective, a question of interest is whether neurodevelopmental outcomes of twins differ from singletons after controlling for covariates. If significant differences in outcomes persist, this would suggest that the twining process itself or something intrinsic to shared life in the womb may be responsible for observed differences. From a clinical perspective, when counseling parents at risk for preterm delivery of twins, it is useful to understand how twin outcomes compare relative to singleton outcomes at the same birth weight or GA. The purpose of this review is to examine the long-term neurodevelopmental outcomes of twins compared with singletons with control for important covariates.