Effect of interpregnancy interval on infant low birth weight: a retrospective cohort study using the Michigan...

To examine the relationship between interpregnancy interval and low birth weight (LBW), using the retrospective cohort design.

We used the maternally linked Michigan livebirth data documented between 1989 and 2000 to evaluate LBW in relation to interpregnancy (i.e., delivery-to-conception) interval, overall and at levels of other reproductive risk factors. We fit separate logistic regression models for pairs of first-second, second-third, third-fourth, and fourth-fifth births to control for confounding.

Of the 565,911 infants identified, 5.5% had LBW. Univariate and stratified analyses showed that the risk for LBW was lowest when the interpregnancy interval was 18-23 months, and increased with shorter or longer intervals. This J-shaped relationship persisted after controlling for all risk factors simultaneously. For example, among the first-second birth pairs, the adjusted odds ratios (AORs) for LBW associated with interpregnancy intervals <6, 24-59, 60-95, and 96-136 months were 1.4 (95% confidence interval [CI] = 1.3-1.5), 1.5 (95% CI = 1.3-1.6), 1.1 (95% CI = 1.0-1.1) and 1.5 (95% CI = 1.3-1.8), respectively, compared with an interval of 18-23 months. Among the second-third birth pairs, the AORs were 1.5 (95% CI = 1.3-1.6), 1.3 (95% CI = 1.2-1.4), 1.1 (95% CI = 1.0-1.1), and 1.6 (95% CI = 1.3-2.0), respectively. Among the third-fourth birth pairs, the AORs were 1.2 (95% CI = 1.1-1.4), 1.3 (95% CI = 1.1-1.5), 1.0 (95% CI = 0.9-1.1), and 1.4 (95% CI = 1.0-2.0), respectively. Among the fourth-fifth birth pairs, the AORs were 1.3 (95% CI = 1.1-1.6), 1.2 (95% CI = 0.9-1.5), 1.1 (95% CI = 1.0-1.4), and 1.3 (95% CI = 0.8-2.3), respectively. The population attributable risk associated with interpregnancy intervals shorter than 18 months or longer than 23 months was 9.4%.

These data suggest that spacing pregnancies appropriately could be used as a strategy for preventing LBW.