Background: No previous study has examined the association between paternal race and birth weight among twins.
Aims: To examine the association of parents' race and birth weight among twins.
Study design: Population-based retrospective cohort study to examine the association between parents' race and birth weight, and its two components, gestation duration and fetal growth among four group of infants: both parents blacks (FB-MB), father white and mother black (FW-MB), father black and mother white (FB-MW), and both parents whites (FW-MW), with "risk-free" analysis to control confounding factors.
Subjects: Twin births (304,466 twins) registered in the United States in 1995-1997.
Outcome measures: The means of birth weight, gestational age, and birth-weight-for-gestational age z score, and rates of very low birth weight (VLBW, <1500 g), very preterm birth (VPTB, <32 weeks of gestation), and fetal growth restriction (FGR, <10th percentile of z score).
Results: Means of birth weight, gestational age, and z score were 2234.0, 2301.9, 2361.1, and 2433.8, and 35.02, 35.24, 35.65, and 35.78, and -0.136, -0.070, -0.039, and 0.063, respectively, and the rates of VLBW, VPTB, and FGR were 14.5, 12.3, 10.5, and 8.1, and 16.0, 13.0, 11.6, and 9.2, and 12.6, 11.1, 10.8, and 8.7, respectively, in the FB-MB, FW-MB, FB-MW, and FW-MW groups. Excluding subjects with various risk factors by risk-free analysis did not change the results.
Conclusions: Both parents' races are important determinants of birth weight, gestational age, and fetal growth among twin pregnancies, with higher effect on gestational age and greater maternal influence than paternal influence.