Introduction: An increasing proportion of women in the US and other countries delay initiation of childbearing until their thirties. Little is known about their subsequent pregnancies, particularly with regard to pregnancy spacing.
Objectives: To determine interpregnancy interval (IPI) patterns, factors associated with IPI among women delaying initiation of childbearing until their thirties, and ascertain if delay in initiation of childbearing is associated with increased likelihood for short interpregnancy interval of less than 6 months.
Methods: A retrospective cohort study was performed using the Missouri maternal linked file for 1978-1997, inclusive. Analysis was limited to mothers aged 20-50 years at first pregnancy, having a first and second pregnancy during the study period; the sample size included 242,559 mother-infant pairs. Analysis strategies included stratified analysis, and multivariable logistic regression. Interpregnancy interval was main outcome variable, and was grouped in seven categories: 0-5, 6-11, 12-17, 18-23, 24-59, 60-119, >or=120 months.
Results: The mean interpregnancy interval was significantly shorter for women delaying start of childbearing (>or=30 years) compared to 20-29 year olds. Observed intervals are 31 (+/-24) months for mothers aged 20-29 years, 25 (+/-17) months for mothers aged 30-34 years, 21 (+/- 14) for 35-39 year olds, and 19 (+/-16) for 40-50 year olds (P < 0.0001). A significant trend for shorter intervals was noted as maternal age at first pregnancy increased (P < 0.0001). Factors associated with interpregnancy interval for women delaying initiation of childbearing included adverse outcome in preceding pregnancy, and low educational status. Mothers aged 35 and above at first pregnancy had increased odds for a second pregnancy following short IPI <6 months; (35-39 years OR = 1.26 95% CI 1.11-1.44; 40-50 OR = 1.91 95% CI 1.13-3.24). Mothers aged 30-34 years have lower odds for short IPI (OR = 0.93 95% CI 0.87-0.99).
Conclusion: First time mothers aged 35 and above have higher odds of having a second pregnancy shortly after their first pregnancy. Given the increasing number of first time mothers aged 35 and above, these findings are of relevance for preconception counseling for this unique population of women.