| Summary: | <h4>Background</h4>The relationship between maternal weight change between pregnancies and premature birth is unclear. This study aimed to investigate whether interpregnancy weight change between first and second, or second and third pregnancy is associated with premature birth.<h4>Methods</h4>Routinely collected data from 2003 to 2018 from one English maternity centre was used to produce two cohorts. The primary cohort (n = 14,961 women) consisted of first and second live-birth pregnancies. The secondary cohort (n = 5,108 women) consisted of second and third live-birth pregnancies. Logistic regression models were used to examine associations between interpregnancy BMI change and premature births adjusted for confounders. Subgroup analyses were carried out, stratifying by initial pregnancy BMI groups and analysing spontaneous and indicated premature births separately.<h4>Results</h4>In the primary cohort, 3.4% (n = 514) of births were premature compared to 4.2% (n = 212) in the secondary cohort, with fewer indicated than spontaneous premature births in both cohorts.<h4>Primary cohort</h4>Weight loss (>3kg/m2) was associated with increased odds of premature birth (adjusted odds ratio (aOR):3.50, 95% CI: 1.78-6.88), and spontaneous premature birth (aOR: 3.34, 95%CI: 1.60-6.98), in women who were normal weight (BMI 18.5-25kg/m2) at first pregnancy. Weight gain >1kg/m2 was not associated with premature birth regardless of starting BMI.<h4>Secondary cohort</h4>Losing >3kg/m2 was associated with increased odds of premature birth (aOR: 2.01, 95%CI: 1.05-3.87), when analysing the whole sample, but not when restricting the analysis to women who were overweight or obese at second pregnancy.<h4>Conclusions</h4>Normal-weight women who lose significant weight (>3kg/m2) between their first and second live pregnancies have greater odds of premature birth compared to normal-weight women who remain weight stable in the interpregnancy period. There was no evidence of association between weight change in women who were overweight or obese at the start of their first pregnancy and premature birth.
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