Faculty Bibliography

OBJECTIVE:Time is an important variable in understanding the recent increase in twin deliveries in the United States. Therefore, this study was designed to estimate the influences of maternal age, period (year) of delivery, and maternal-birth-year cohort on trends in rates of twin deliveries. METHODS:United States natality data were used to assess trends in twin pregnancies resulting in live births. This age-period-cohort analysis included 7, 5-year maternal-age groups (15-19 through 45-49 years), 6 twin delivery periods (1975, 1980, 1985, 1990, 1995, and 2000), and 12, 5-year maternal birth cohorts (1926-1930 through 1981-1985). The independent effects of maternal age, twin delivery period, and maternal birth cohort on twin delivery rates for blacks and whites were modeled using Poisson regression techniques. RESULTS:Our study assessed 95,042 blacks and 401,989 whites with twin deliveries. Twin deliveries increased by 46% for blacks and 62% for whites from 1975 to 2000, with the largest increase occurring in the year 2000. For blacks, maternal age had the strongest impact on the increasing twin delivery rates, followed by period of delivery. For whites, the greatest effect was due to period of delivery, followed by maternal birth year cohort and, lastly, maternal age. CONCLUSION/CONCLUSIONS:Our data confirm the importance of nature's biologic contribution of maternal aging to twin delivery rates, but suggest that recent changes in the environment surrounding pregnancy (nurture) also influence twin delivery rates. The relative contributions of biologic versus environmental influences appear to differ among blacks and whites.

OBJECTIVE:To estimate the pattern of maternal vascular reactivity in normal and high-risk pregnancies using postocclusion brachial artery diameter. METHODS:Prospective, longitudinal study of 44 low-risk singleton pregnancies and 28 high-risk pregnancies, defined as pregestational diabetes (n = 7), chronic hypertension (n = 4), twin gestation (n = 6), and a previous history of preeclampsia, fetal growth restriction, or vascular disease (n = 11). During each trimester, the brachial artery was ultrasonographically imaged above the antecubital crease. Brachial artery diameter was measured and then occluded for 5 minutes using an inflated blood pressure cuff. Changes in brachial artery diameter at 1 minute after occlusion were expressed as percent change from baseline and were compared across trimesters for both low-risk and high-risk groups, adjusting for potential confounders. RESULTS:Brachial artery diameters were increased after occlusion in every trimester for all groups. For low-risk women, the degree of postocclusion brachial artery dilatation was similar in the first and second trimesters, but was lower in the third trimester. In the first trimester, low-risk women had significantly greater brachial artery diameter increases at 1 minute compared with high-risk singleton pregnancies (19% compared with 12%; P <.001). Compared with low-risk women, pregnancies complicated by pregestational diabetes or chronic hypertension had significantly smaller 1-minute brachial artery diameter changes in the first trimester (7.0 +/- 0.5%, P <.001), whereas twin gestations had greater brachial artery responses (22.9 +/- 6.0%, P <.001). Women with previous preeclampsia or vascular disease had responses similar to low-risk women. CONCLUSION/CONCLUSIONS:Maternal vascular reactivity as assessed by postocclusion brachial artery dilatation decreases in the third trimester in both low-risk and high-risk women. In addition, singleton pregnancies at high risk for preeclampsia display decreased brachial artery reactivity compared with low-risk women.

OBJECTIVE:Although twin stillbirth rates have declined substantially over the past two decades, the contribution of changes in obstetric interventions to reducing twin stillbirths has not been quantified. METHODS:We carried out a retrospective cohort study of twin live births and stillbirths in the United States between 1989 and 1999 (n=1,102,212). Changes in the rate of stillbirth (> or =22 weeks) before and after adjustment for changes in labor induction, Cesarean delivery and sociodemographic factors were estimated through ecological logistic regression analysis. This analysis was based on aggregating data by each state within the United States. RESULTS:Between 1989 and 1999, rates of labor induction and Cesarean delivery among twin live births increased by 138% (from 5.8% to 13.8%) and 15% (from 48.3% to 55.6%), respectively. These changes were accompanied by a 43% decline in the stillbirth rate between 1989 and 1999 (from 24.4 to 13.9 per 1000 fetuses at risk). After excluding births weighing < 500 g, rates of labor induction among twins at 22-27 weeks', 28-33 weeks' and > or =34 weeks' gestation increased by 95%, 131% and 127%, respectively, between 1989 and 1999. Cesarean delivery rates also increased by 55%, 29% and 2% in these same gestational age categories. The 48% (relative risk (RR) 0.52, 95% confidence interval (CI) 0.49-0.55) decline in stillbirth rate between 1989-91 and 1997-99 was reduced to a 25% (RR 0.75, 95% CI 0.72-0.79) decline after adjustment for changes in labor induction and Cesarean delivery. The decline in the rate of twin stillbirths was larger at later gestational ages (at > or =32 and > or =34 weeks) where the largest absolute increases in labor induction rates were observed. CONCLUSIONS:The use of Cesarean delivery and especially labor induction for twin pregnancies has increased substantially in the United States over the last decade and these changes have been associated with a large decline in the rate of stillbirth among twins.

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.

BACKGROUND:During the last 2 decades, the rate of low birthweight has increased, as has the rate of preterm delivery, among both whites and blacks. Examination of causes for these secular trends has focused largely on changes in the distributions of maternal age and, less commonly, on birth cohort. Little is known as to how age, period, and birth cohort interact on trends in small births at term. METHODS:The U.S. natality files were used to assess trends in term (>/=37 weeks gestation) small-for-gestational age (SGA) births for 7 5-year maternal age groups (15-19 through 45-49 years), 6 delivery periods (1975, 1980, 1985, 1990, 1995, and 2000), and 12 5-year maternal birth cohorts (1926-1930 through 1981-1985). SGA births were defined as sex-specific birthweight below the 10th percentile for gestational age based on 1995 livebirths in the United States. Logistic regression models were fit to determine the independent effects of age, delivery period, and birth cohort on term SGA trends, separately for blacks and whites. RESULTS:Between 1975 and 2000, term SGA births declined by 23% (from 21% to 16%) among blacks and by 27% (from 12% to 9%) among whites. Term SGA births declined with increasing age up to 30-34 years, but increased among older women. Within strata of maternal age, the risk also declined with later maternal birth cohorts, among both blacks and whites. The strongest influence on SGA trends was from maternal age, followed by maternal birth cohort, and lastly by delivery period. In general, for any combination of age, period, and birth cohort, blacks showed 1.5- to 2-fold higher rates of term SGA than whites. CONCLUSIONS:The persistence of strong maternal age effects on risk of term SGA births suggests that the effect of age is at least partly the result of biologic factors. Term SGA trends were generally consistent for blacks and whites, although the magnitude of difference in the risks for combinations of age, period, or mother's birth cohort was higher among blacks than whites.

OBJECTIVE:To examine the association between intratriplet birth weight discordance, fetal and neonatal mortality, and smallness for gestational age. METHODS:The 1995-1997 Centers for Disease Control and Prevention's Matched Multiple Birth file was used for this analysis. Birth weight discordance was calculated as the difference in birth weight between the largest and the smallest triplet's weight and expressed as percentage of the largest triplet's weight. For the middle-weight triplet, we also used the largest triplet's weight as a reference in calculating percentage birth weight discordance, which was then grouped into quintiles. RESULTS:Among 15,511 triplet live births and fetal deaths (at least 20 weeks' gestation), 35% had less than 10% birth weight discordance, 19.3% had 10-15%, 16.4% had 15-21%, 15.2% had 21-29%, and 14.1% had 29% or more. After controlling for confounders, the risk of fetal death associated with quintile V was significantly higher than that associated with quintile I for smallest (odds ratio [OR] 10.88; 95% confidence interval [CI] 4.87, 26.56), middle (OR 22.6; 95% CI 11.05, 46.3), and largest (OR 2.41; 95% CI 1.01, 5.89) triplets. Smallest and middle triplets in quintile V were more likely than quintile I triplets to be born small for gestational age (OR 26.0; 95% CI 17.1, 39.9 for smallest, and OR 13.4; 95% CI 8.01, 22.3 for middle). Birth weight discordance quintile was not associated with smallness for geatational age among largest triplets nor consistently with neonatal mortality among smallest, middle, or largest triplets. CONCLUSION/CONCLUSIONS:Increasing birth weight discordance was associated with increased risk of fetal death and smallness for gestational age. A birth weight discordance threshold of at least 29% should alert obstetricians for appropriate decision making.

This study was undertaken to examine the relationship between paternal and maternal age differences and adverse perinatal outcomes in the United States. Data were obtained on singleton pregnancies delivering at >or=20 weeks gestation in the United States in 1995-97 from the National Center for Health Statistics data sets. Adverse perinatal outcomes that were evaluated included fetal death rate (>or=20 weeks), preterm delivery <37 weeks and small-for-gestational-age (SGA) births (birthweight <10th centile for gestational age and corrected for sex). Age difference was defined as paternal minus maternal age. The analysis included 8995274 pregnancies (11.3% blacks, 88.7% whites). An increase in fetal death rate, preterm delivery and SGA births was noted among white women who were older than their male partners. For black mothers older than their partners, there was an increase in fetal death rate when the women were <20 years old, but a decrease in fetal death rate when >35 years old. Neither rates of preterm delivery nor SGA births were increased much for black women with varying parental age differences. This demonstrates that race and maternal age both contribute to the effects of parental age difference on adverse perinatal outcomes.

OBJECTIVE:To determine whether the presence of labor affects infant mortality among small-for-gestational-age (SGA) infants. METHODS:Data were derived from the United States national linked birth/infant death data sets for 1995-97. Singleton SGA live births in cephalic presentation delivered at 24-42 weeks' gestation were included. Mortality rates for SGA infants exposed and unexposed to labor were compared, and relative risks (RR) were derived using multivariable logistic regression models, after adjusting for potential confounding factors. RESULTS:Of 986 405 SGA infants, 87.4% were exposed to labor. Infants exposed to labor at 24-31 weeks had greater risks of dying during the early neonatal period (RR 1.79-1.86). Decreased risks of late and postneonatal death were observed at all gestational ages in the presence of labor. CONCLUSIONS:Exposure to labor is associated with an increased risk of early neonatal death among SGA infants, especially at gestational ages below 32 weeks. Future randomized trials are warranted to determine the optimal obstetric management of these high-risk infants.

As genetic research and technology continues to expand, carrier testing for an increasing number of single gene disorders is becoming available. Tay-Sachs disease and cystic fibrosis are two common recessive conditions with large-scale health implications. Tay-Sachs disease was the first genetic disorder for which community-based screening efforts were utilized and has provided a foundation for the development of other screening programs. Cystic fibrosis testing, on the other hand, has additional complexities and the implementation of population-based screening has been under debate. The many issues (technical, educational, social, psychological and economical) which must be considered as preconceptional and prenatal genetic screening is incorporated into clinical practice are discussed here in the context of Tay-Sachs disease and cystic fibrosis.

Uterine rupture can occur at any time throughout gestation. We present a woman with a previous Cesarean section followed by an abdominal pregnancy. In her next pregnancy, complete uterine rupture resulted in an emergency laparotomy. This case is unique in that it gives insight into the variable presentations of uterine rupture and the risks associated with prior Cesarean sections.