Normal-weight women who lose more than 3 kg/m2 in body mass between the start of their first and second live pregnancies are more than three times as likely to deliver prematurely in the latter pregnancy, compared to normal-weight women whose weight remains stable in the interpregnancy period. This is the main finding of an analysis of the United Kingdom-based SLOPE (Studying Lifecourse Obesity PrEdictors) project in the journal PLOS One.
However, there was no evidence of a connection between weight change in women who were overweight or obese at the start of their first pregnancy and premature birth in the second pregnancy. Weight stability in the analysis was defined as -1 and +1 body mass index (BMI) units kg/m2 between the start of the first pregnancy and the start of the second pregnancy.
“The overall aim of the SLOPE project is to study maternal and early-life predictors of childhood obesity,” said Nisreen Alwan, MBCHB, PhD, the principal investigator of the analysis and an associate professor of public health at the University of Southampton, United Kingdom. Dr. Alwan said one of the research themes of the SLOPE project is studying interconception health to identify how change in modifiable factors between successive pregnancies is linked to important health indicators from birth onwards. “This is to inform the design of effective interventions during this period which supports women and families in optimizing their current and future health,” she said.
The analysis used data from 2003 to 2018 of roughly 15,000 women who received antenatal care and lived in Southampton and surrounding parts of Hampshire, United Kingdom, during at least two of their pregnancies. Women who were within the normal weight range (18.5 to 24.9 kg/m2) at the start of their first pregnancy and lost significant weight between their first and second pregnancies—but not enough to place them in the underweight category—were at significantly higher risk of a premature birth than those who maintained a stable weight: adjusted odds ratio (aOR) = 3.5; 95% confidence interval (CI): 1.8 to 6.9. “This relationship is not necessarily causal,” Dr. Alwan told Contemporary OB/GYN. “For example, ill general health or stress can lead to both weight loss and premature birth.”
Only 1% of normal-weight women lost more than 3 kg/m2 between their two pregnancies. “Overall, losing weight was much less common than gaining weight between pregnancies, with about half of women gaining weight (1 kg/m2or more) between their first and second pregnancies compared to under 16% losing weight during that period,” Dr. Alwan said.
But there was no evidence of a link between weight loss and premature birth in women with overweight or obesity at the start of their first pregnancy, or in women who gained weight between pregnancies after taking into account confounding factors.
A separate study from the SLOPE project demonstrated that 16% of babies born to women who gained substantial weight (3 kg/m2 or more) between pregnancies were large-for-gestational-age (LGA) compared to 12% of babies born to women who lost weight or remained at a stable weight between pregnancies.
“Women who gained a significant amount of weight by the start of their second pregnancy were more likely to be smokers and unemployed, with lower educational attainment and a longer interpregnancy interval, compared with those who maintained a stable weight between pregnancies,” Dr. Alwan said. “The study also found that overweight women who lost weight (1 kg/m2or more) after an LGA birth in their first pregnancy were 31% less likely to have recurrent LGA in their second pregnancy.”
The findings of the analysis could help doctors, midwives and other health professionals identify women who may have greater odds of premature and LGA birth, by linking a woman’s weight at the start of her previous pregnancy, according to Dr. Alwan.
The research is supported by an Academy of Medical Sciences and Wellcome Trust grant. Dr. Alwan also receives research support from the National Institute for Health Research (NIHR) Southampton Biomedical Research Centre and the NIHR Applied Research Collaboration, Wessex, United Kingdom.