Racial Differences in the Influence of Interpregnancy Interval on Fetal Growth.

Objectives Assess the influence of maternal race on the association between interpregnancy interval (IPI) and risk of small for gestational age (SGA) and large for gestational age (LGA) births. Methods Statewide population-based cohort study of 380,520 singleton births. We calculated risk of SGA and LGA births following IPIs of 0 to <6, 6 to <12, 12 to <24 (referent), 24 to <60 months, and ≥60 months, by maternal race after adjustment for confounding influences. Results The highest risk for SGA among white women followed short IPI of 0 to <6 months [adjRR 1.14 (95 % CI 1.08-1.21)], and long IPI ≥ 60 months [adjRR 1.37 (95 % CI 1.31-1.43)]. Only long IPI ≥ 60 months increased SGA risk in black women [adjRR 1.22 (95 % CI 1.13-1.32)]. LGA risk in white women was lowest with shortest and longest IPIs, 0 to <6 [adjRR 0.80 (95 % CI 0.76-0.84)] and ≥60 months [adjRR 0.68 (95 % CI 0.66-0.70)]. The crude risk of LGA was directly proportional to longer IPIs in black women. However, after adjusting for confounding effects of age, obesity, excessive gestational weight gain, and gestational diabetes, the effect was reversed to reduced risk following long IPI ≥ 60 months [adjRR 0.82 (95 % CI 0.74-0.91)], similar to that of white women. Conclusions In black and white women, an interpregnancy interval of 1-2 years is associated with optimal fetal growth. In addition to birth spacing, addressing modifiable factors such as pre-pregnancy BMI, monitoring gestational weight gain, and control of gestational diabetes in black women may help optimize fetal growth.