The US study, using birth records from the Ohio Department of Health, looked at 454,716 live births from women with two or more pregnancies over a six year period. The researchers looked at the influence of inadequate birth spacing on the duration of the subsequent pregnancy.
The study defined short interpregnancy interval (IPI) as time from the immediate preceding birth to subsequent conception of the next pregnancy. Researchers categorized the women with short IPIs into two groups, IPI less than 12 months and IPI 12-18 months, comparing them to women who were considered to have an optimal IPI of 18 months or more.
Results showed that mothers with shorter IPIs were more likely to give birth prior to 39 weeks gestation when compared to women with optimal birth spacing. Following a short IPI of less than 12 months, 53.3% of women had delivered before 39 weeks, compared to 37.5% of women with an optimal IPI. Birth after the estimated due date (more than 40 weeks) occurred less often in women with short IPI of less than 12 months, 16.9% compared to 23.1% for a normal IPI.
Furthermore the rate of preterm birth before 37 weeks gestation was higher in women who conceived after a short IPI of less than 12 months. They were more than twice as likely to give birth before 37 weeks compared to pregnancies following an optimal IPI (20.1% vs. 7.7% respectively).
The study also looked at racial groups. Figures showed that black mothers more frequently had shorter IPIs compared to non-black mothers (less than 12 months, 3.3% vs. 1.9% and 12-18 months, 13.2% vs. 10.1%).
Moreover, the rate of preterm birth was also higher in black mothers with short IPI of less than 12 months, 26.4% compared to 8.7% for non-black mothers.
While women who conceived following an optimal IPI (18 months or more) had the lowest rates of preterm birth, black women still showed a higher rate of overall preterm births (11.3%) compared to non-black mothers (6.8%), suggesting that maternal ethnicity was also a significant predictor of preterm birth despite optimal birth spacing.
The study's authors recommended women try for an "optimal" birth spacing of 18 months or longer between pregnancies.
Image: Pregnant woman holding a baby, via Shutterstock