A lthough the association between interpregnancy interval and the risk of adverse fetal and neonatal outcomes has been examined for vaginal delivery, the effects of birth interval on the safety and efficacy of vaginal birth after cesarean delivery (VBAC) are less well known. This study examined whether a short or long interpregnancy interval is associated with a risk of increased uterine rupture and other maternal morbidities in women attempting a VBAC.The database for the study involved a secondary analysis of a multicenter, record-based, retrospective cohort study of pregnant women with at least one prior cesarean that involved teaching university and community hospitals and nonteaching community hospitals. The cohort included 25,005 patients who delivered between 1995 and 2000 at 17 participating hospitals in the Northeastern United States. This subanalysis taken from that database included only women who decided to try a VBAC. Short interpregnancy interval cut-off points were <6, <12, and <18 months between prior delivery and conception dates; the long interval cut-off was Z60 months. The primary outcome of uterine rupture was defined as a uterine scar separation determined at laparotomy that was preceded by a nonreassuring fetal heart rate pattern, maternal signs or symptoms of acute blood loss, or hemoperitoneum to distinguish it from asymptomatic uterine dehiscence. Secondary outcomes were composites of maternal major morbidity and maternal requirements for blood transfusions.Of the 25,005 women with prior cesarean sections, 55% attempted a VBAC (n = 13,706). Data on the interval between cesarean sections was not available in 375, leaving 13,331 records available for analysis. The rates of short interpregnancy interval for <6 months, 6 to 11 months, and 12 to 17 months were 2.2% (n = 286), 8.3% (n = 1109), and 13.1% (n = 1741); that for the long interval was 19.7% (n = 2631). Uterine rupture occurred in 128 cases, for a rupture rate of 0.9% in those who attempted VBAC. Women with an interval of <6 months had a significant increase in uterine rupture rate, with an absolute risk of 2.7%, nearly a 3-fold increase relative to those with birth intervals >6 months. This short-interval group also had a significant 2-fold increase in the composite morbidity rate (absolute risk of 4.2%) and a 3-fold risk of receiving a blood transfusion compared with the women in the other interval groups. These increases in risk persisted when the data were adjusted for relevant confounders such as number of prior cesarean and vaginal deliveries, delivery gestational age, cigarette use, whether labor was induced, and maternal age, race, and parity. When the short interval cutoffs of <12 months and <18 months were used, the risks of uterine rupture, composite morbidity, and blood transfusion were not statistically different from the long interval group. The long interval was not an independent risk factor for any of the three outcomes. The overall failure to achieve a VBAC was about 23% among all patients and did not vary significa...