Study population
Out of 981,353 registered births in 2010-2014, we identified 4728 nulliparous women with a singleton pregnancy that ended in PTB between 16+0-27+6 weeks. From these, 2434 pregnancies were induced or ended with an elective cesarean section and were excluded. In total, 2294 prior births were included for linkage (Figure 1).
After linkage with a cohort of 1,782,991 primiparous births, 1285 of the included 2294 women with prior sPTB could be linked with a subsequent pregnancy that ended in a birth after 16 weeks of gestation. Prior pregnancies complicated by an antenatal diagnoses of intra-uterine fetal death (IUFD) (n=127) or fetal congenital abnormalities (n=159) (together n=274, numbers do not add up due to overlap in diagnosis) were excluded from the prior sPTB pregnancies after linkage. Ultimately, we were able to include the perinatal outcomes of 1011 linked pregnancies in this study. The flow chart of inclusion and exclusion per year is shown in figure 1.
Patient characteristics of the index pregnancy are shown in table 1 and are shown separately for women with prior sPTB between 16+0-19+6, 20+0-23+6 and 24+0-27+6 weeks of gestation. The number of patients in each group was respectively 342, 346 and 323. The proportion of women with a western ethnicity differed significantly between groups, with the highest proportion of women with an ethnicity other than a western ethnicity in the group with birth between 16+0 and 19+6. Despite an almost equal number of male fetuses (n=511) and female fetuses (n=500) in the overall group, the proportion of male fetuses being born between 16+0-19+6 weeks (35.7%) was significantly lower compared to 20+0-23+6 (59,5%) and 24+0-27+6 (56,7%) weeks (p<0.001). Maternal age, ART involvement and low SES did not differ significantly between the sPTB groups.