Outcomes
The recurrence rate of PTB < 37 weeks in women with prior birth between 16+0-19+6 was 19.0% (95% CI 15.0%-23.6%). Women with prior birth between 20+0-23+6 weeks had a recurrence rate of PTB <37 weeks of 29.5% (95% CI 24.7% - 34.6%) and women with prior birth between 24+0-27+6 weeks had a recurrence rate of PTB < 37 weeks of 27.6% (95% CI 22.8% - 32.8%). The recurrent rates for PTB < 24, < 32 and < 37 weeks with 95%-confidence intervals are shown in table 2. For the three groups, odds for PTB < 24 and PTB < 32 weeks did not differ significantly between groups (Supplementary table 1). Figure 2 shows the risk of recurrent preterm birth per week of gestational age groups in prior sPTB birth.
Interpregnancy interval Compared to the reference group with an interpregnancy interval of 13-36 months, an interval of 0-3 months was found to be associated with significantly increased odds for PTB < 32 weeks (OR 2.3 95% CI 1.4-3.7) and PTB < 37 weeks (OR 1.8 95% CI 1.2-2.6). The odds per interval groups are shown in table 3. Compared to women with prior birth at a viable gestational age > 24 weeks, women with previous pre-viable birth between 16+0-19+6, 20+0-23+6 weeks were more likely to have a short interpregnancy interval < 3 months (14.2% vs respectively 19.6% and 28.0%). (supplementary table 2)