Classification of small-for-gestational age births: weight-by-gestation standards of second birth conditional on the size of the first

Percentiles of weight-by-gestational age were constructed for first and second births, based on linked sibship-data from the Medical Birth Registry of Norway. Standards were made for weight-by-gestational age of second births conditional on whether the first birth was small-for-gestational age (SGA) or large-for-gestational age (LGA). These standards were compared with the conventional, cross-sectional standard of all second births. The relevance of the conditional standards was assessed on the basis of perinatal mortality, using logistic regression analyses. When applying cross-sectional standards of second births, more than 30% of the births following a SGA first birth were classified as SGA, compared with only 1.7% following an LGA first births. The overall risk for a perinatal loss in second births following a SGA first birth was twice that among second births following a LGA first birth. When second births were themselves categorised as SGA or non-SGA using the cross-sectional standards, the mortality among the SGA second births was such that the risk was 4 to 5 times higher following LGA first births compared with SGA first births. When conditional standards were applied to define SGA among second births, the risk relation between the subgroups (defined by classification of first birth) corresponded to the observed overall risk pattern. An unconditional SGA classification conceals important differences between clinically distinct subgroups.     

Birthweight, preterm births and neonatal mortality in Belgium and the United States

Summary. Belgium is known to have a lesser low birthweight rate and a lower infant mortality rate than the United States. We used previously unpublished data to show that beneath this comparison lies a more complicated picture. Singleton live birth certificates for 1986-67 were analysed. Despite a lower mean birthweight in Belgium (3360 g) than in the United States (3420 g), Belgium had fewer (4.9%) low birthweight infants than the US (5.9%) because of fewer preterm births (4.4 vs. 9.3%). Consistent with the excess of preterm births in the US, the residual distribution of birthweight was smaller in Belgium (2.2% vs. 3.1%). Whereas neonatal mortality was 4.8/1000 in Belgium and 5.6/1000 in the US, birthweight-specific neonatal mortality was higher in Belgium. The challenge for Belgium is to improve the survival of newborns regardless of their birthweight. In the US, the task is to eliminate the excess of small preterm infants.     

Birthweight and perinatal mortality among singletons and twins in north-eastern Tanzania

BACKGROUND: Low birthweights as well as high perinatal mortality rates are common in most African populations. Little is known, however, about how low birthweight corresponds with higher mortality rates within African populations. Twins are known to have lower birthweights and higher perinatal mortality rates than singletons. If lower birthweights represent higher perinatal risk per se, small twins within a population with generally lower birthweights should have critically increased risks. METHODS: In total, 15,255 births in a Tanzanian hospital during 1999-2006 were analysed to determine birthweight distribution and examine perinatal mortality rates (including stillbirths and neonatal deaths within 24 hours) by birthweight in twins and singletons. Referral births from outside the district where the hospital was situated were excluded from analysis. RESULTS: The mean birthweight for births within an estimated normal distribution was 3172 g, with a standard deviation of 462 g. The overall perinatal mortality rate was 43.9 per 1000 births (95% confidence interval: 40.7-47.2). Perinatal mortality rates among twins and singletons were 91.0 and 41.1 per 1000 babies respectively, corresponding to a relative risk of 2.2 (95% confidence interval: 1.7-2.8). The birthweight distribution for twins was shifted to lower birthweights. Twins had a generally lower birthweight and an excess of extremely small births as compared to singletons. The increased mortality rate for twins appeared to be independent of birthweight. CONCLUSIONS: The two-fold increased risk of perinatal death for twins was observed across the whole birthweight distribution, and very small twins appeared to have an excess perinatal risk that was almost similar to that of larger twins.     

Birthweight of offspring and mortality of parents: the Jerusalem perinatal study cohort

PURPOSE: We sought to examine the association between birthweight in offspring and mortality in their parents. Distinguishing between risks of outcomes in mothers from fathers potentially provides clues as to the relative roles of genetic versus nongenetic mechanisms underlying these associations. METHODS: We studied total and cause-specific mortality in a population-based cohort of 37,718 mothers and 38,002 fathers whose offspring were delivered in West Jerusalem during 1964-1976, after an average follow-up of 34.12 years. RESULTS: Hazard models controlling for sociodemographic and lifestyle characteristics indicated a U-shaped relationship between offspring's birthweight and overall mortality, deaths from coronary heart disease, circulatory and other non-neoplastic causes in their mothers. Greater rates of mortality from coronary heart disease were observed among mothers who gave birth to babies with low (hazard ratio [HR], 2.13; 95% confidence interval [CI], 1.40-3.25) and high birthweight (HR, 1.98; 95% CI, 1.36-2.88), as compared with mothers whose offspring weighed 2500-3999 g at birth. Adjustment for maternal pre-eclampsia slightly attenuated these results. Multivariate models indicated a negative linear relationship (HR, 0.95; 95% CI, 0.91-0.99) between offspring's birthweight and overall mortality in their fathers. Unlike the association in mothers, the relation was noted primarily with deaths from "other causes." CONCLUSIONS: Birthweight of offspring is associated with parental mortality although the relation differs for fathers and mothers. These findings broaden previous observations that intra-uterine events have long-term consequences for adult health and support the need to explore genetic and/or environmental mechanisms underlying these associations.     

The epidemiology of perinatal mortality in multiple births.

The epidemiology of perinatal mortality in multiple pregnancies was investigated from data on 16,831 multiple births from New York City''s computerized vital records for 1978-1984. Twins had a sixfold higher rate of neonatal death and a threefold higher rate of fetal death during labor than had singleton infants. Much of this excess mortality can be explained by the lower birthweight distribution in twins: between 1,001 and 2,500 grams twins had birthweight-specific death rates equivalent to or substantially less than singletons. However, in infants of normal birthweights, twins had more than three times the mortality risk of singletons. For twins in vertex presentation between 1,001 and 3,000 grams, cesarean section did not appreciably reduce neonatal mortality risk. For twins in vertex presentation who weighted more than 3,000 grams the neonatal mortality rate was more than four times higher in vaginal deliveries than in cesarean sections (exact p = 0.034). Efforts to prevent intrapartum and neonatal mortality in multiple births should aim at reducing the incidence of low birthweight twins. More research is needed on the etiology of perinatal problems in normal birthweight twins (greater than or equal to 2,501 grams), especially on the effects of different modes of delivery.     

Birthweight and perinatal mortality: III. Towards a new method of analysis

Perinatal mortality is closely related to birthweight. We propose a model that summarizes this relationship and provides a basis for the analysis of perinatal mortality. The components of this model are the frequency distribution of birthweight and the curve of weight-specific mortality. Taken together, these two curves completely describe perinatal mortality for a given population. The perinatal mortality of two populations can be meaningfully compared by plotting each weight-specific mortality curve relative to its own birthweight distribution. By this means, the excess mortality in one population can usually be expressed as the sum of two excess mortalities--one that occurs uniformly over the whole birthweight distribution, the other due to an increased number of small births. To illustrate this method, we analyse differences between white and black infants. We find that the excess mortality of black infants is chiefly due to an excess of small black births, but also to higher mortality over all (adjusted) birthweights. In contrast to other methods of analysis, the proposed method does not assume a priori that a difference in the mean birthweight of two populations is the cause of any difference in perinatal mortality. Furthermore, the proposed method is unbiased; in particular, it is preferable to direct or indirect standardization for birthweight, previously shown to be biased.     

Birthweight and perinatal mortality: paradoxes,social class,and sibling dependencies

BACKGROUND: Birthweight distributions among second-born infants depend on the birthweights of older siblings, with implications for weight-specific perinatal mortality. We wanted to study whether these relations were explained by socioeconomic levels, and to study time trends in a situation with decreasing perinatal mortality rates. METHODS: Births in the Norwegian Medical Birth Registry from 1967 to 1998 were linked to their mothers through their national identification numbers. The study population was 546 688 mothers with at least two singletons weighing >/==" BORDER="0">500 g at birth. Weight-specific perinatal mortality for second-born siblings in families with first-born siblings in either the highest or the lowest birthweight quartile was analysed. Maternal education and cohabitation status were used as measures of socioeconomic level. RESULTS: For all 500-g categories below 3500 g, mortality rates were significantly higher among second-born infants with an older sibling in the highest rather than the lowest weight quartile. This pattern was the same across three educational levels. The exclusion of preterm births did not change the effect pattern. A comparison of perinatal mortality among second siblings in terms of relative birthweight (z-scores) showed a reversal of the relative risks, although these were only significantly different from unity for the smallest infants. Conclusion The crossover in weight-specific perinatal mortality for second siblings by weight of first sibling is largely independent of socioeconomic level, and is not weakened by the decreasing perinatal mortality rates in the population over time. Family data should be taken into consideration when evaluating the risk of adverse pregnancy outcome relating to weight.     

Birthweight and relationship with infant, child and adult mortality in the Jerusalem perinatal study

There is growing evidence that several chronic adult diseases, such as coronary heart disease and stroke, can result from events occurring in fetal life. The aim of this study was to examine the relation between birthweight and all-cause mortality in young adults. We studied total mortality in a population-based cohort of 80 936 offspring born in Jerusalem in 1964-76. During an average follow-up of 28.8 years 2 324 984 person-years were contributed and 2092 deaths occurred. Overall, in both genders, the univariable and the multivariable Cox-proportional hazard models indicated a strong negative relationship between birthweight and total mortality, mostly because of infant deaths. At ages 1-14 birthweight seemed unrelated to all-cause mortality. In males aged 15+, birthweight was again a significant predictor of death (Hazard ratio (HR) = 0.88, 95% confidence interval (CI) [0.78, 0.99], for 1 standard deviation (SD) increase in birthweight). The analysis by categories suggested a general decreasing of the risk of mortality with increasing birthweight (HRs = 1.0, 1.02, 0.85, 0.77, 0.57 for those belonging to birthweight groups of < 2500 g, 2500-2999 g, 3000-3499 g, 3500-3999 g and > or = 4000 g, respectively). In females aged 15+ there was a J-shaped relation between birthweight and mortality but these associations were not statistically significant. These findings add to a growing body of evidence that events during intrauterine life have remote consequences for adult health and underline the need to consider gender differences.     

Birthweight of single livebirths and weight specific early neonatal mortality in Hungary and Norway

Comparisons of birthweights of single livebirths in Hungary and Norway reveal distributions to have similar shapes; however, in the case of Hungary the distribution is shifted to the left, i.e. towards lower weights. In the registration of pregnancy outcomes, almost identical definitions are applied in the two countries, and the observed difference in distributions of birthweights is taken to reflect that Norwegian livebirths, are on average about 300 g heavier than Hungarian livebirths. Employing the method of analysis of birthweight and perinatal mortality suggested by Wilcox & Russell, it can be demonstrated that the proportions of births in the residual distributions of birthweights in the two countries are of the same magnitude and that the relative differences in first week mortality risks are similar for all birthweights. These results are taken to support the conclusion that to use a cut-off point of 2500 g in defining low birthweight, which will result in a two fold higher proportion of such infants in Hungary compared to Norway, is unwarranted, as it will falsely convey the impression of relatively more obstetric and paediatric problems in Hungary.     

Birth weight and perinatal mortality: the effect of gestational age.

BACKGROUND. The strong association between birth weight and perinatal mortality is due both to gestational age and to factors unrelated to gestational age. Conventional analysis obscures these separate contributions to perinatal mortality, and overemphasizes the role of birth weight. An alternative approach is used here to separate gestational age from other factors. METHODS. Data are from 400,000 singleton births in the Norwegian Medical Birth Registry. The method of Wilcox and Russell is used to distinguish the contributions to perinatal mortality made by gestational age and by relative birth weight at each gestational age. RESULTS. Gestational age is a powerful predictor of birth weight and perinatal survival. After these effects of gestational age are controlled for, relative birth weight retains a strong association with survival. CONCLUSIONS. Current public health policies in the United States emphasize the prevention of low birth weight. The present analysis suggests that the prevention of early delivery would benefit babies of all birth weights.     

Relationship of cigarette smoking and social class to birth weight and perinatal mortality among all births in Britain, 5-11 April 1970.

The joint associations of maternal cigarette smoking and social class on perinatal outcome were studied in the 1970 British birth cohort (British Births). Whereas smoking was much more frequent among women in social classes III, IV, and V, there was little difference in the birthweight decrement associated with smoking across class. Perinatal mortality, however, was increased only among smokers in the manual social classes. Thus whereas the offspring of more privileged smokers were not protected from intrauterine growth retardation, they did not suffer from increased perinatal mortality. Observations of other populations suggest a possible nutritional mediation of this protective effect.     

Maternal weight status and term birth weight in first and second adolescent pregnancies.

The relationship of maternal weight status to birth weight was evaluated retrospectively for the first and second pregnancies of 72 younger (age 12-15 years at first conception) and 80 older (age 16-19 years at first conception) low-income adolescents (76% black, 24% white). Mean birth weight increased during the second pregnancies of both groups (277 g and 132 g, respectively). Multiple regression analysis (controlling for potentially confounding variables) indicated a positive relationship between second and third trimester rate of maternal weight gain (kg/week) and birth weight for younger adolescents during their second pregnancy (p = 0.014), and for older adolescents during their first pregnancy (p = 0.047). Mean birth weight increased with each increase in maternal weight-for-height (W/H) category near term from the lowest (< 100% of standard) to the highest (> or = 140% of standard), for both age groups during both pregnancies. Multiple regression analysis indicated that among older adolescents birth weight increased 4.2 +/- 2.0 g (X +/- SE) for each 1% increase in maternal W/H near term in their first pregnancy (p = 0.038) and 7.1 +/- 1.8 g for each 1% increase in maternal W/H near term in their second pregnancy (p = 0.0003). Among younger adolescents these relationships, while in the same direction, were not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)