Interpregnancy Interval and Subsequent Severe Maternal Morbidity: A Population-based Study from California over 16 years.

Interpregnancy interval (IPI) associates with adverse perinatal outcomes, but its contribution to severe maternal morbidity (SMM) remains unclear. We examined the association between IPI and SMM, using data linked across sequential pregnancies to women in California 1997-2012. Adjusting for confounders measured at the index pregnancy (i.e. the first in a pair of consecutive pregnancies), we estimated adjusted risk ratios (aRRs) of SMM related to the subsequent pregnancy. We further conducted within-mother comparisons and analyses stratified by parity and maternal age at the index pregnancy. Compared to 18-23 months, IPI<6 months had same risk for SMM in between-mother comparison (aRR=0.96, 95%CI 0.91, 1.02) but lower risk in within-mother comparison (aRR=0.76, 95% confidence interval (CI) 0.67, 0.86). IPI 24-59 months and IPI≥60 months associated with increased risk of SMM in both between-mother (aRR=1.18, 95%CI 1.13, 1.23 and aRR=1.76, 95% CI 1.68, 1.85 respectively) and within-mother comparisons (aRR=1.22, 95%CI 1.11, 1.34 and aRR=1.88, 95% CI 1.66, 2.13 respectively). The association between IPI and SMM did not substantially differ by maternal age and parity. Longer IPI was associated with increased risk of SMM, which may be partly attributed to interpregnancy health.