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.