Association Between Gestational Weight Gain and Obstetric Anal Sphincter Injury

Area of Science:

• Obstetrics and Gynecology
• Maternal-Fetal Medicine
• Public Health

Background:

• Gestational weight gain (GWG) is a critical factor in pregnancy outcomes.
• Prepregnancy body mass index (BMI) influences GWG and associated risks.
• Obstetric anal sphincter injury (OASI) is a significant complication of vaginal birth.

Purpose of the Study:

• To investigate the association between GWG relative to Institute of Medicine (IOM) guidelines and OASI risk.
• To examine these associations across diverse prepregnancy BMI categories.
• To explore the mediating role of fetal birth weight in the GWG-OASI relationship.

Main Methods:

• Retrospective population-based cohort study using US National Vital Statistics System natality files (2021-2023).
• Analysis of singleton, term, vaginal births.
• Logistic regression and restricted cubic spline regression to assess GWG (below, within, above IOM guidelines) and OASI risk across BMI categories; mediation analysis for birth weight.

Main Results:

• OASI occurred in 1.2% of births; rates were higher with GWG above recommendations (1.3%).
• GWG above and within IOM guidelines increased OASI odds across most BMI categories (average, overweight, obesity classes I & II).
• GWG below recommendations showed a protective effect only for average-weight individuals; birth weight mediated the GWG-OASI association across all BMI groups.

Conclusions:

• GWG above IOM guidelines is linked to increased OASI risk across most BMI categories, except underweight and class III obesity.
• GWG below guidelines offered protection only at average weight.
• The observed associations between GWG and OASI are substantially mediated by fetal birth weight.