Statistical analysis
Following the linkage between nulliparous women with previous sPTB and
primiparous women with an ongoing pregnancy after 16 weeks of gestation,
the baseline characteristics of the first pregnancy were described and
tested for the three spontaneous PTB groups of the index population;
chi-square for categorical variables and anova for continues variables.
Logistic regression analysis was performed to assess the association
between the gestational age at prior birth and the PTB pregnancy
outcomes of the subsequent pregnancy. PTB rates are expressed per 100
births. Odds ratios (OR) and 95% confidence intervals (C.I.) were
calculated separately for the three outcome groups (PTB <24
weeks, PTB < 32 weeks and PTB <37 weeks) in the
subsequent pregnancy, with prior birth between 24+0 – 27+6 weeks as the
reference. Analysis was adjusted for maternal characteristics (age,
ethnicity, low SES), neonatal characteristics (fetal sex) and obstetrics
characteristics (interpregnancy interval).
A separate analysis was conducted to assess the association between the
interpregnancy interval, gestational age at prior birth and the
recurrent risk. The interpregnancy interval was subdivided into groups
of 0-3, 4-6, 7-12, 13-36 and >37 months, with 13-36 months
as the reference.
Linkage and analysis were performed using SAS, version 9.4.