What are the fetal growth patterns of singletons, twins, and triplets in the United States?

A retrospective study was undertaken to determine if premature infants had a higher incidence of intrauterine growth retardation than term infants did. If premature labor is significantly associated with intrauterine growth retardation, then defining intrauterine growth retardation with a population-specific postnatal birth weight for gestational age curve would underestimate the incidence in preterm infants. Data for the year 1990 were used to construct a postnatal birth weight for gestational-age curve. This curve was then used to analyze 1991 birth weight data and to determine the incidence of intrauterine growth retardation (< 10th percentile) at each week of gestation. Infants were also classified as having intrauterine growth retardation on the basis of an additional postnatal birth weight for gestational-age curve and two antenatal ultrasonic estimated fetal-weight-for-gestational-age curves. Analysis of the 1991 delivery data indicated that both ultrasonography curves showed a significant decrease in the incidence of intrauterine growth retardation with advancing gestational age, whereas the postnatal curves did not. The results give full support to previous reports that suggest intrauterine growth retardation is more common in preterm than in term infants and are consistent with the hypothesis that intrauterine growth retardation is significantly related to premature birth.

Infant mortality among US black and white twins and singletons was compared for 1960 and 1983 using the Linked Birth/Infant Death Data Sets from the National Center for Health Statistics. Both twin and singleton infant mortality rates showed impressive declines since 1960 but almost all of the improvement in survival for both twins and singletons was related to increased birth weight-specific survival rather than improved birth weight distribution. One-half of white twins and two-thirds of black twins weighed less than 2,500 g at birth, and 9% of white twin births and 16% of black twin births were in the very low (less than 1,500g) birth weight category. In 1983, twin infant mortality rates were still four to five times that of singletons. However, twins had a survival advantage in the 1,250-3,000 g range, which persisted after adjustment for gestational age. Cause-specific mortality among twins was considerably higher for every major cause of death: twin mortality risks due to newborn respiratory disease, maternal causes, neonatal hemorrhage, and short gestation/low birth weight were six to 15 times that of singletons. The lowest twin-to-singleton mortality ratios observed were for congenital anomalies and sudden infant death syndrome with relative risks twice that of singletons. The data underscore the need to develop effective strategies to decrease infant mortality among twins.

Our purpose was to evaluate factors associated with the best intrauterine growth and lowest morbidity among twins ("ideal twin pregnancy"). A historic prospective study of 163 twin births was performed. Ten models were formulated with multiple regression and multivariate logistic regression. In the models of birth weight, gestations of 28 to 36 and 39 to 41 weeks, black race, > or = 15% discordancy, and smoking were all significant negative factors. The pattern of early low weight gain (< 0.85 pounds per week before 24 weeks) and late low weight gain (< 1.0 pound per week after 24 weeks) was negatively associated with all eight models of intrauterine growth. The best intrauterine growth and lowest morbidity is achieved earlier for twins than for singletons. Using length of stay and growth retardation criteria, nearly 70% of "ideal" twin pregnancies were between 35 and 38 weeks. In addition, poor weight gain and poor patterns of weight gain were associated with all measures of intrauterine growth and adverse pregnancy outcomes.