649: Spontaneous preterm birth rates among unselected women in their first and second pregnancy

It is known that preterm birth (PTB), defined as birth <37 weeks of gestation, is associated with an increased risk of PTB in a subsequent pregnancy. However, in contrast to pre-eclampsia, it is unknown whether in a fixed population spontaneous PTB rates change between the first and the second pregnancy. The aim of this study was to compare PTB rates in unselected women with two subsequent singleton pregnancies. We used a population based longitudinal linked dataset of the National Perinatal Registry the Netherlands. We studied data of all 232,389 nulliparous women who had two consecutive singleton pregnancies ending between 22-43 weeks of gestation in the period 1999 and 2007. Women whose pregnancy was complicated by congenital abnormalities, stillbirth or a primary caesarean section were excluded. We compared the spontaneous PTB rate in the first and second pregnancy in terms of delivery <37, <34 and <30 weeks. The number of women with spontaneous PTB rates <37 weeks decreased from 11,535 (7.9%) to 6,560 (3.7%) (RR 0.57; 95% CI 0.49 to 0.58), while the number of women suffering severe spontaneous PTB (<30 weeks) decreased from 726 (0.49%) to 384 (0.21%) (RR 0.53; 95% CI 0.49-0.60). Among 726 women who delivered initially spontaneously <30 weeks, 225 (31%) delivered again preterm, of which 39 (5%) <30 weeks. Among 10,809 women who delivered initially spontaneously between 30 and 37 weeks, 1,874 (17%) delivered again preterm, of which 69 (0.6%) <30 weeks. Out of the 217,278 women who delivered initially at term, 5,540 (2.5%) spontaneously delivered preterm in the next pregnancy, of which 385 (0.1%) <30 weeks. In identical women, the spontaneous PTB rate decreased by almost 50% between the first and second pregnancy, and was seen for both severe and for mild PTB rates. As the observed effect also occurred in women that delivered between 34-37 weeks, it cannot be explained by a treatment effect. This makes point at a strong role for an immunologic contribution in the pathway to PTB.