Temporal changes in the utilization of amniocentesis for prenatal diagnosis by women of advanced maternal age, 1976-1983

Utilization of amniocentesis for prenatal diagnosis because of the indication of advanced maternal age (greater than or equal to 35 years at delivery) was examined in the population of B.C. over an 8-year period (1976-1983). As of 1983, approximately one out of three eligible women (greater than or equal to 35 years at delivery) is having the test. In the older group (greater than or equal to 38 years at delivery) approximately one out of two eligible women is having the test. The data do not indicate that the proportion of eligible women having amniocentesis has reached a plateau; utilization is continuing to increase. It appears that prenatal diagnosis is an ethically acceptable alternative for a large proportion of the population.     

Older maternal age and infant mortality in the United States

We used data from the National Infant Mortality Surveillance project to examine the effect of older maternal age on infant mortality for the 1980 United States birth cohort. The 1,579,854 births and 14,591 deaths of singletons who were black or white and whose mothers were 25-49 years of age were included. Direct standardization was used to calculate birth-weight-adjusted relative risks of neonatal and postneonatal mortality, using the birth weights of infants with maternal age 25-29 as the standard. We found that the risk of infant mortality was nearly equal for infants born to mothers 25-29 and 30-34 years of age; infants born to mothers 35-39 years of age were at a slightly elevated (18% higher) risk, and those born to mothers 40-49 years of age were at a much more elevated (69% higher) risk. Among whites, the higher neonatal mortality associated with a maternal age of 35-39 was mostly due to an increased prevalence of low birth weight; among blacks, it was due to higher birth-weight-specific risks. Neither white nor black postneonatal mortality risks were much elevated until a maternal age of 40-49, and this last elevation was mostly due to higher birth-weight-specific risks. These findings suggest that infertility and fetal mortality aside, and considering only the effect on infant mortality, it is relatively safe for women to postpone childbearing into their middle, and perhaps late, thirties.     

The impact of prenatal diagnosis on the occurrence of chromosome abnormalities

From the public health point of view, several formal attempts have been made to measure the impact of prenatal diagnosis (PND) on the incidence of Down's Syndrome (DS), but the results have varied widely. The impact of PND (reduction in the birth rate of chromosomally abnormal neonates) is related to utilization rates but quantitative estimates of this have not been established. In a three-year (1981-1983) total population study from Queensland, Australia, we present results to measure the impact of a voluntary PND programme on the birth incidence of DS, and also other chromosomally abnormal births. Utilization rates for the PND service were 15.5 per cent in that population of mothers 35 years and over. Numbers and rates of all cases of chromosomal abnormalities are presented, subclassified by type of diagnosis--either by PND or by clinical diagnosis after birth. For the total population, 7.3 per cent of cases of DS were detected prenatally, and 15.4 per cent of all chromosome abnormalities. (A method for measuring the impact of PND is described.) Using this in conjunction with our demographic data, we estimate that with a 15 per cent utilization rate of PND by older mothers, 14 per cent of DS births can be prevented in this age group, or a 5 per cent overall reduction can be achieved if mothers of all ages are considered. One index--the ratio of the percentage of DS births which are preventable compared with the population utilization rates of PND--has potential for widespread use. Queensland data for this ratio is 0.34, a figure consistent with that from other studies. Thus a 3.5 per cent drop in the overall DS birth rate may be expected for each 10 per cent increase in the utilization rates of PND for mothers of 35 years and over. A diagram is presented which may serve as a model for improved data collection and better impact estimates in the future.     

Trends in asthma mortality in Taiwan, 1981-2000.

BACKGROUND AND PURPOSE: The prevalence and mortality rates of asthma in many countries have increased in the past 2 decades. The purpose of this study was to determine the trends of asthma mortality in Taiwan from 1981 through 2000. METHODS: Data from the National Health Statistics for Taiwan identifying asthma [International Classification of Disease (ICD)-9 code 493] as the underlying cause of death were collected. Age-specific and age-adjusted mortality rates of asthma were calculated and the 20-year trends were analyzed. RESULTS: The crude mortality rate of asthma decreased from 8.17 to 4.53 per 100,000 general population, and the age-adjusted mortality rate decreased from 8.17 to 2.37 per 100,000 during the 20-year period. This trend of decreasing mortality was most obvious for age groups older than 35 years, i.e., from 4.05 to 1.20 per 100,000 in those aged 35 to 54 years and from 47.43 to 13.44 per 100,000 in those aged 55 to 74 years. The mortality rate for asthma in those aged 15 to 34 years remained the same, around 0.40 to 0.35 per 100,000 population. For those below 15 years of age, the mortality rates for asthma fluctuated between 0.30 and 0.10 per 100,000 population. CONCLUSIONS: The mortality rate of asthma has steadily decreased in Taiwan in the past 20 years, especially in the age groups above 35 years.     

Oral contraception and circulatory risks

The longterm prospective study of health effects of oral contraceptives (OCs) conducted by the Royal College of General Practitioners compared 23,000 OC users with 23,000 controls matched for age and marital status. As of 1981, 55 deaths attributable to circulatory problems had occurred in ever-users of OCs, ciompared to 10 in controls, giving a relative risk of 4.2 for OC users. No relation was found between duration of use and mortality risk among users, although mortality risks were greater at all durations of use than for nonusers. Parity was related to mortality risk among users but not among controls. The most significant factors affecting the relation between pill use and circulatory risk were age and smoking. Among users who smoked, the risk ratios were 3.4 for those aged 25-34, 4.2 for those aged 35-44, 7.4 for those aged 45 and over, and 5.1 for the entire group. Among nonsmoking pill users, the ratios were 1.6 for those aged 25-34, 3.3 for those aged 35-44, 4.6 for those 45 and over, and 3.2 for the total sample. Among smokers, the rates of excess deaths were 1 in 10,000 for users aged 15-34, 1 in 2000 for those aged 35-44, and 1 in550 for those aged 45 and over. Among nonsmokers, the rates were 1 in 50,000 users for those aged 25-34, 1 in 6700 for those aged 35-44, and 1 in 2500 for those aged 45 and over. The majority of deaths were attributed to ischemic cardiac problems and to sub-arachnoid hemorrhages, and risks appeared to be elevated in former as well as current users. The total incidence of circulatory effects in former users appears to be elevated only for cerebrovascular disorders, but the suggestion of residual effects requires further study before conclusions can be drawn. Smokers who developed cardiovascular or cerebrovascular problems were at 2-3 times greater risk of dying than were other women. The percentages of fatal cases of ischemic heart and cerebrovascular diseases were 22.8% among ever-users who smoked, 10.9% among controls who smoked, 7.5% among nonsmoking ever-users, and 6.6% among nonsmoking controls. A comparison of users of 5 different kinds of pills, 3 containing the same dose of ethinyl estradiol (EE) and varying doses of norethisterone acetate, and 2 containing the same dose of EE and varying doses of levonorgestrel, indicated that the number of arterial disorders increased significantly with increasing dosages of progestin.     

Maternal factors associated with delivery of infants with birthweight less than 2000 grams in a low socioeconomic population

Women of low socioeconomic status (SES) are known to have a higher incidence of low birthweight (LBW) infants who are a major source of neonatal mortality and morbidity. SES, however, does not define a homogeneous population. If prenatal programs could be oriented specifically toward women of higher risk within the low SES population, they should be more effective. Within our uniformly low SES population, we compared mothers who had given birth to infants weighing less than 2000 g with mothers of infants weighing over 3000 g in order to identify factors associated with a LBW birth. We found medical factors, including hypertension and infection during pregnancy, demographic factors, including race, marital status, and lack of prenatal care, and lifestyle factors, including drug and alcohol abuse, to be more common among women giving birth to infants weighing less than 2000 g. These factors can be used to develop more effective intervention programs for low SES populations.     

Recurrent small for gestational age birth: perinatal risks and outcomes

The risk of recurrent small for gestational age birth, as well as maternal and fetal-neonatal characteristics associated with recurrence, was examined in 174 mothers of consecutively delivered small for gestational age infants followed through an additional 240 livebirths. There was a twofold and fourfold increase in the risk for small for gestational age birth after one and two small for gestational age births, respectively. Although an intervening average for gestational age birth decreased the risk of recurrence, these women remained at increased risk over the general population. Given the history of a previous small for gestational age birth, perinatal risks and outcomes considered individually would not improve the prediction of recurrence. However, the significantly higher frequency of these variables, considered as a group, among mothers with recurrent small for gestational age birth suggests an association with underlying maternal disease, for example, chronic hypertension, substance use and abuse, more severe fetal-neonatal compromise, and recurrent small for gestational age birth. Recurrent small for gestational age birth should initiate a search for persistent, underlying maternal disease.     

Risk Factors and Birth Outcomes Associated with Teenage Pregnancy: A Canadian Sample

Study ObjectiveTo examine the extent to which socioeconomic status, mental health, and substance use are associated with teenage pregnancies in Southwestern Ontario (SWO), and whether these pregnancies are at an elevated risk for adverse birth outcomes, after controlling for medical, behavioral, and socioeconomic status factors.DesignRetrospective cohort study using perinatal and neonatal databases.SettingTertiary care hospital in SWO.ParticipantsWomen residing in SWO who gave birth to singleton infants without congenital anomalies between 2009 and 2014. Teenage pregnancies (19 years of age or younger) were compared with pregnancies of women 20-34 years and 35 years or older.InterventionsNone.Main Outcome MeasuresLow birth weight (LBW), very LBW, term LBW, preterm birth, very preterm birth, low and very low Apgar score, and fetal macrosomia.ResultsOf 25,263 pregnant women, 1080 (4.3%) were 19 years of age or younger. Approximately 18% of teenage mothers lived in socioeconomically disadvantaged neighborhoods, compared with 11% of mothers aged 20-34 and 9% of women 35 years of age or older (P < .001). Teenage mothers had higher rates of depression during pregnancy (9.8%) than mothers 20-34 years (5.8%) and those 35 years of age or older (6.8%; P < .001). Young mothers self-reported higher tobacco, marijuana, and alcohol use during pregnancy than adult mothers (P < .001). Teenage pregnancy increased the risk of a low Apgar score (adjusted odds ratio, 1.56; 95% confidence interval, 1.21-2.02), but was not associated with other birth outcomes after adjusting for covariates.ConclusionTeenage pregnancy is associated with a higher risk of socioeconomic disadvantage, mental health problems, and substance use during pregnancy, but is largely unrelated to adverse birth outcomes in SWO.     

Twins born over weekends: are they at risk for elevated infant mortality?

Objective

To assess the impact of the day of birth on twin mortality in a population sample.

Methods

We analyzed weekend versus weekday twin births from the United States national twin birth data for the periods 1989–2002. We computed adjusted hazard ratios (HR) and 95?% confidence intervals (CI) to assess the association between infant mortality and weekday of birth using the Cox proportional hazards model.

Results

The crude rates for all types of mortality were found to be significantly higher for twins born on weekends than on weekdays. After adjustment, only post-neonatal mortality risk was higher on weekends as compared to weekdays [Hazards ratio (HR)?=?1.19, CI: 1.04, 1.36]. Twins of white mothers were at greater risk for neonatal death (HR?=?1.16, CI: 1.08, 1.24) but were less likely to experience post-neonatal death (HR?=?0.68, CI: 0.64, 0.76) as compared to twins of black mothers. We found an interaction between maternal age and weekday of birth. Twins born on weekends to teenage mothers (age <18) had a 35?% greater risk for neonatal death (HR?=?1.35, CI: 1.06, 1.71) while those born on weekends to older mothers did not show elevated risk for any of the mortality indices.

Conclusion

Increased risks for post-neonatal death are significantly higher amongst twins born on weekends as compared to weekdays. Further research is required to identify the detailed differences in structure and procedures that result in the disadvantage associated with weekend birth.     

Relative risks of oral contraceptive usage

Women seeking safe, reliable contraception must weigh the risks associated with such methods with their benefits. To make this assessment process more meaningful, the authors have prepared a chart that sets forth the risks of oral contraceptive (OC) use by different user factors (e.g., age, cigarette smoking) in proportion to the risks inherent in a variety of sporting activities and types of accidents. Mortality rates reported for OC use include those due to the method and those due to pregnancies resulting from method failure. Scrutiny of this chart reveals that the risk of death associated with swimming, boating, and automobile use exceeds the risk of death for nonsmokers who use OCs up to 34 years of age (1.6 deaths/100,000 population). OC use by women 15-24 years of age who smoke carries a lower mortality risk (3.0/1000) than scuba diving or falling, while OC users 25-34 years old who smoke are more likely to die as a result of homicide, suicide, or automobile accidents than they are as a result of their use of OCs (mortality rate, 10.2/1000). On the other hand, OC use by smokers 35-44 years of age is associated with a mortality rate of 84.5/1000, which is a higher risk than that associated with the sport of hang-gliding.     

Prognosis for twins with birth weight less than 1500 gm: the impact of cesarean section in relation to fetal presentation

The purpose of this study of twins weighing less than 1500 gm was to evaluate the impact of cesarean section on intrapartum and neonatal mortality, as well as on cerebral palsy and mental retardation. National data held at the Medical Birth Registry was used for identification of cases. The original medical records were retrieved for 862 such twins born between 1973 and 1983. Twins with cerebral palsy and/or mental retardation born between 1973 and 1980 were identified by questionnaires to all rehabilitation centers for disabled children, offices for the Provision of Care for the Mentally Retarded, and to all local Boards of Education throughout Sweden. The analyses, including calculation of relative risk and 95% confidence interval, were performed after stratification for birth weight (250 gm classes) and period of delivery (1973 to 1976, 1977 to 1980, 1981 to 1983). The cesarean section rate increased from 7.7% (1973 to 1976), and 40.5% (1977 to 1980) to 68.9% (1981 to 1983). Concomitantly, intrapartum and neonatal mortality decreased markedly (51.7% to 29.1%) particularly for twin II but to a much lesser extent for twin I. The relative risk for intrapartum and neonatal mortality (vaginal/abdominal birth) did not increase significantly for twin I in vertex presentation (relative risk 2.0, 95% confidence limits 0.9 to 4.3), for twin I in breech presentation (relative risk 1.8, 95% confidence limits 0.7 to 4.3), for twin II in vertex presentation (relative risk 0.6, 95% confidence limits 0.2 to 1.6), or for twin II in breech presentation (relative risk 1.5, 95% confidence limits 0.7 to 3.0). The rate of cerebral palsy and/or mental retardation was 8.8% during 1973 to 1976 and 8.0 during 1977 to 1980 (chi 2 = 0.1, p greater than 0.05). For twins born in breech presentation the handicap rate in the first period (cesarean section rate 6.0%) was the same as in the second period (cesarean section rate 59.6%). The analysis failed to reveal any significant impact of abdominal birth on the fetal outcome for low-birth-weight twins, even when fetal presentation was taken into consideration.     

Risk of a Down syndrome live birth in women 45 years of age and older

OBJECTIVES: To determine the risk of a Down syndrome (DS) live birth for women 45 years of age and over. METHODS: A meta-analysis of data from five published articles, 13 EUROCAT congenital anomaly population registers and two unpublished sources. RESULTS: Information was available on the number of DS live births occurring amongst 13,745 live births to women 45 years of age and over. Information was also available on DS pregnancies diagnosed prenatally that were subsequently terminated. These pregnancies were adjusted for expected fetal loss to estimate the number of live births that would have occurred in the absence of prenatal diagnoses, when a total of 471 DS live births were estimated to have occurred. The risk of a DS birth did not increase for women 45 years of age and over. The average risk was 34 per 1000 births (95% CI: 31-37). CONCLUSION: The risk of a DS live birth for women 45 years of age and over is considerably lower than has often been previously assumed. The most likely explanation is that women of this age are more likely to miscarry DS pregnancies than younger mothers.     

Birth outcomes after prenatal exposure to antidepressant medication

OBJECTIVE: The purpose of this study was to examine prospectively the incidence of congenital anomalies and neonatal complications after prenatal exposure to antidepressant medication. STUDY DESIGN: Birth outcomes were obtained from a review of obstetric and neonatal records of 138 women who were treated with selective serotonin reuptake inhibitor antidepressant medications (SSRIs) during pregnancy. RESULTS: The incidence of congenital anomalies in this study was 1.4%, comparable to general population rates. Rates of low birth weight and preterm births were low, occurring in 2.9% and 6.5% of births, respectively. The low birth weight infants had been exposed to relatively high doses of fluoxetine (40-80 mg/d) throughout pregnancy. Average maternal weight gain in pregnancy was comparable across the three major medication categories (fluoxetine, paroxetine, sertraline). CONCLUSION: After prenatal use of selective serotonin reuptake inhibitor antidepressant medications, neonatal complications and congenital anomalies appear to occur within general population rates. However, maternal use of high doses of fluoxetine throughout pregnancy may be associated with a risk for low birth weight.     

The effect of maternal demographic factors on infant mortality rates. Summary of the findings of the Louisiana Infant Mortality Study. Part I.

Birth and infant death certificates for Louisiana in 1972 were matched and reviewed to identify groups particularly in need of close medical support and counselling during the identify groups particularly in need of close medical support and counselling during the life phases of preparation for reproduction, gestation, and parenthood. From all recorded birth and infant death certificates for 1972 (69,556 birth and 1,541 death certificates) infant, neonatal, and postneonatal mortality rates were computed for maternal demographic (intrinsic patient physiocal and life-style characteristics) factors including age, race, parity by age cohort, education, and legitimacy. Excessive infant deaths were found among illegitimate offspring, except in mothers less than 15 years of age or nonwhites over 35 years of age; the less educated; those having too many children too soon; those of low birth weight; and whites relying on Charity Hospitals. An increased mortality rate was seen with nonwhites and with the younger and older mothers.     

Debendox revisited

Summary. Two further studies re-examine the findings by Golding e t al . (1983) in Oxford that mothers of children born with oral clefts had been more frequently prescribed Debendox (Bendectin) than had the mothers of matched controls. In Cardiff, 86 283 total births between 1965 and 1979 included 139 infants with oral clefts. Detailed information was obtained about 93 of these infants and matched controls. The relative risk of the birth of an infant with an oral cleft to women prescribed Debendox within 69 days of the first day of their last men-strual period was 0.64 (95% confidence limits 0.12–3.34). No increased effect was found amongst women who smoked. In Aberdeen, 17 640 livebirths between 1976 and 1979 included 31 children with oral clefts. Compared with the non-exposed group the risk of a birth of an infant with an oral cleft to a woman prescribed Debendox in early pregnancy was 0.37 (95% confidence limits 0.09–1.47). These results do not confirm the previous findings in Oxford of a possible association between exposure to Debendox in early pregnancy and the occurrence of oral clefts in the child.     

Debendox revisited

Two further studies re-examine the findings by Golding et al. (1983) in Oxford that mothers of children born with oral clefts had been more frequently prescribed Debendox (Bendectin) than had the mothers of matched controls. In Cardiff, 86 283 total births between 1965 and 1979 included 139 infants with oral clefts. Detailed information was obtained about 93 of these infants and matched controls. The relative risk of the birth of an infant with an oral cleft to women prescribed Debendox within 69 days of the first day of their last menstrual period was 0.64 (95% confidence limits 0.12-3.34). No increased effect was found amongst women who smoked. In Aberdeen, 17 640 livebirths between 1976 and 1979 included 31 children with oral clefts. Compared with the non-exposed group the risk of a birth of an infant with an oral cleft to a woman prescribed Debendox in early pregnancy was 0.37 (95% confidence limits 0.09-1.47). These results do not confirm the previous findings in Oxford of a possible association between exposure to Debendox in early pregnancy and the occurrence of oral clefts in the child.     

The perinatal effects of delayed childbearing

OBJECTIVE: To determine if the rates of pregnancy complications, preterm birth, small for gestational age, perinatal mortality, and serious neonatal morbidity are higher among mothers aged 35-39 years or 40 years or older, compared with mothers 20-24 years. METHODS: We performed a population-based study of all women in Nova Scotia, Canada, who delivered a singleton fetus between 1988 and 2002 (N = 157,445). Family income of women who delivered between 1988 and 1995 was obtained through a confidential linkage with tax records (n = 76,300). The primary outcome was perinatal death (excluding congenital anomalies) or serious neonatal morbidity. Analysis was based on logistic models. RESULTS: Older women were more likely to be married, affluent, weigh 70 kg or more, attend prenatal classes, and have a bad obstetric history but less likely to be nulliparous and to smoke. They were more likely to have hypertension, diabetes mellitus, placental abruption, or placenta previa. Preterm birth and small-for-gestational age rates were also higher; compared with women aged 20-24 years, adjusted rate ratios for preterm birth among women aged 35-39 years and 40 years or older were 1.61 (95% confidence interval [CI] 1.42-1.82; P < .001) and 1.80 (95% CI 1.37-2.36; P < .001), respectively. Adjusted rate ratios for perinatal mortality/morbidity were 1.46 (95% CI 1.11-1.92; P = .007) among women 35-39 years and 1.95 (95% CI 1.13-3.35; P = .02) among women 40 years or older. Perinatal mortality rates were low at all ages, especially in recent years. CONCLUSION: Older maternal age is associated with relatively higher risks of perinatal mortality/morbidity, although the absolute rate of such outcomes is low.     

Maternal age and incidence of low birth weight at term: a population study

A total of 184,567 singleton live births with gestational ages of 40 weeks were examined from the 1980-1984 Illinois birth certificate data to determine the independent effect of maternal age on the incidence of low birth weight at term. The incidence is highest in mothers less than 17 years of age (3.2%) and gradually declines with advancing maternal age to reach 1.3% in women aged 25 to 34 years. It increases to 1.7% for those greater than 35 years of age. To separate out the independent effect of maternal age on the incidence of low birth weight infants at term, the presence of other maternal factors, such as race, education, parity, marital status, and prenatal care, were adjusted by use of a series of multiple logistic regression analyses. All of these analyses consistently demonstrated that the adjusted risk for low birth weight at term is the lowest in teenagers and increases with advancing maternal age. These results indicate that the high incidence of this factor in young mothers apparently reflects their poor sociodemographic and prenatal care status. Advancing maternal age is associated with a decreased potential for fetal growth, possibly reflecting biologic aging of maternal tissues and systems or the cumulative effects of disease.     

Risk factors for low birth weight among first-time mothers in southern Taiwan.

BACKGROUND AND PURPOSE: Low birth weight (LBW) greatly affects infant development, the family, and health care financial systems. Nonetheless, the risk factors for LBW in Taiwan have not been investigated. The purpose of this study was to determine the risk factors for LBW in Kaohsiung County. METHODS: In this cross-sectional study, we recruited 1,147 first-time mothers, including all adolescent mothers (< 20 years old) who gave birth during the period from June 1994 through May 1995 and all adult mothers (> or = 20 years old) who gave birth in January and February 1995. The subjects were interviewed during home visits or by telephone by public health nurses in each township. Logistic regression analysis was used to identify the risk factors for LBW in adolescent and adult mothers. RESULTS: The prevalence of LBW in the study population was 6.2%, ranging from 7.6% in adolescent mothers to 4.9% in adult mothers. Univariate analysis showed that mothers who had low pregravid weight (< 45 kg), infrequent prenatal visits (< 10), anemia, low gestational weight gain (< 10 kg), or habits of alcohol consumption or cigarette smoking were more likely to give birth to LBW infants. In logistic regression analysis, after controlling for covariates, the significant risk factors for LBW were low gestational weight gain (< 10 kg) and low pregravid weight (< 45 kg) for adolescent mothers. Infrequent prenatal visits (< 10) was the only significant risk factor for adult mothers. CONCLUSIONS: In this study, the risk factors for LBW among adolescent and adult mothers were different. This suggests that programs for prevention of LBW should be tailored according to maternal age, begin before conception, and continue with enhanced surveillance during prenatal visits.     

Further defining the risks confronting twins

OBJECTIVE: We report the absolute and relative risks for neonatal and infant death, low and very low birth weight, and delivery at <33 and <35 weeks' gestation in twin pregnancy stratified by maternal race and age, as well as gender pair combinations of the twins. STUDY DESIGN: Data on 324,141 twin infants were obtained from the 1985 to 1988 U.S. Linked Birth/Infant Death Data Sets. In this observational cohort study, we analyzed the outcomes of 138,779 twin pregnancies of white and black women that ended with the delivery of two live-born infants. RESULTS: Rates for the aforementioned outcomes are increased for black infants, for male-male pairs compared with male-female pairs (with female-female pairs being intermediate), and for young mothers. For male-male twins born to young (≤22 years old) black women, relative risks range from 2.1 for both pair members being low birth weight (<2500 gm) to 5.0 for both pair members dying in infancy, when male-female pairs born to older (≥28 years) white women served as the reference group. CONCLUSIONS: Although all twin pregnancies are at higher risk than singletons, risk is influenced by maternal race and age, as well as gender pair combination. These analyses provide useful information for counseling women pregnant with twins. Furthermore, they suggest that failure to consider variations in baseline risk may have seriously flawed studies evaluating prophylactic interventions in twin pregnancy. (Am J Obstet Gynecol 1996;175:1522-8.)