Spontaneous and iatrogenic preterm birth rates among unselected women in three consecutive pregnancies.

OBJECTIVE: To assess the risk of sPTB and iPTB in women with three consecutive singleton pregnancies and the impact of the outcome of the 1st and 2nd pregnancy on the (recurrent) PTB risk in the 3rd pregnancy.

STUDY DESIGN: A nationwide retrospective cohort study using the population based longitudinal linked dataset of the Netherlands. We included all nulliparous women with three consecutive singleton pregnancies ending between 22 and 44 weeks of gestation between 1999 and 2009. We excluded congenital abnormalities and stillbirths. We compared the incidence of sPTB and iPTB in the three pregnancies (<37, <34 and <30 weeks). Logistic regression analysis was performed to predict PTB in the 3rd pregnancy, adjusting for maternal age, fetal gender, socio-economic status, hypertension, interpregnancy interval, artificial reproductive technology, and small for gestational age. Analyses were also performed stratified by prior PTB subtype, gestational age and combined outcome of the 1st and 2nd pregnancy.

RESULTS: We studied 52,978 women. PTB occurred in 7.0%, 3.7% and 3.4% in the 1st, 2nd and 3rd pregnancy, respectively. The outcome of the 2nd pregnancy is more predictive for PTB in the 3rd pregnancy then the outcome of the 1st pregnancy (sPTB aOR7.3 (95%CI 6.3-8.4) and iPTB (aOR 5.9 (95% CI 4.5-7.9) in 2nd pregnancy vs. sPTB aOR 3.0 (95% CI 2.6-3.4) and iPTB aOR 2.7 (95% CI 2.1-3.4) in the 1st pregnancy). In the prediction of sPTB in the 3rd pregnancy, sPTB in the 2nd pregnancy is most predictive (aOR8.2 (95% CI 7.1-9.6) and for prediction iPTB in the 3rd pregnancy, iPTB in the 2nd pregnancy is most predictive (aOR12.1 (95% CI 8.5-17.2).

CONCLUSION: We studied a population with three subsequent singleton deliveries within 10 year. The incidence of PTB decreased with 50% from the 1st to the 2nd pregnancy, to then stay relative stable in the 3rd pregnancy. Compared to PTB in the 1st pregnancy, PTB in the 2nd pregnancy is more predictive for the occurrence of PTB in the 3rd pregnancy.