This study examines the effects of birth order and interpregnancy interval on birthweight, gestational age, weight-for-gestational age, infant length, and weight-for-length in a sample of 2063 births from a longitudinal study in the Philippines. First births are the most disadvantaged of any birth order/spacing group. The risks associated with short intervals (< 6 months) and high birth order (fifth or higher) are confined to infants who have both attributes; there is no excess risk associated with short previous intervals among lower-order infants, nor for high birth order infants conceived after longer intervals. This pattern is observed for all five birth outcomes and neonatal mortality, and persists in models that control for mother's age, education, smoking, family health history and nutritional status. Since fewer than 2% of births are both short interval and high birth order, the potential reduction in the incidence of low birthweight or neonatal mortality from avoiding this category of high-risk births is quite small (1-2%).
PIP: A study of 2063 liveborn infants born in the Metro Cebu region of the Philippines shows considerable differentials in pregnancy outcomes based on birth order and length of preceding interpregnancy interval. First born infants fared worse than infants of higher birth orders and any birth spacing group. Specifically, they had lower mean birth weight (2852 vs. 3052 g), weight for gestational age (less than the 10th percentile birth weight for gestational age, 20.5% vs. 12.8%), weight for height (14.8% low Rohrer's index vs. 10.5%), shorter gestation (38.5 weeks vs. 38.8 weeks), and height (48.9 cm vs. 49.3 cm). These deficits were still significant even when the researcher controlled for mother's age, education, nutritional status, and smoking during pregnancy. First borns were at 2.1 greater risk of neonatal mortality than other births, which could be partially accounted for by lower birth weight and shorter gestation. Infants conceived within 6-12 months of the last birth had modest deficits in all pregnancy outcomes, except gestation length, when compared to those conceived at greater intervals. Low birth weight and shorter gestations explained much of the excess mortality of infants born after short interpregnancy intervals. No excess risk of low birth weight, prematurity, length, intrauterine growth retardation, low birth weight for length, or neonatal mortality existed for short previous intervals among low order infants or for high birth order infants conceived after longer intervals. Just 2% of all infants were births of high birth order (= or 5) and short interval ( 6 months). Thus, the attributable risk of neonatal mortality was less than 2%. First births were a subgroup with a much higher attributable risk (e.g., 16% for low birth weight), but they cannot be avoided as a way to improve infant health outcomes.