# Relationship between Maternal Body Composition during Pregnancy and Newborn Birth Weight in Japan

## TL;DR

This study explores how changes in Japanese mothers' body composition during pregnancy relate to their newborns' birth weight, with findings varying based on pre-pregnancy BMI.

## Contribution

The study identifies how maternal body composition changes and their associations with birth weight differ across BMI categories in Japanese women.

## Key findings

- Fat-free mass in normal and overweight groups positively correlates with birth weight.
- Fat mass gain in underweight and normal groups negatively correlates with birth weight.
- Obese groups show no significant associations between body composition and birth weight.

## Abstract

This study aimed to investigate the changes in maternal body composition during pregnancy in Japanese women and the relationship between maternal body composition and newborn birth weight using pre-pregnancy body mass index (BMI) in all trimesters.

A total of 1,851 pregnant Japanese women were enrolled in this study. Body composition was measured using TANITA MC-190EM. The associations between newborn birth weight and maternal BMI, fat mass (FM), fat-free mass (FFM), total body water (TBW), muscle mass (MM), FM gain, FFM gain, and weight gain were evaluated.

The participants’ age and pre-pregnancy BMI were 34.1 years and 21.4 kg/m2, respectively. Among the patients, 13.4%, 73.0%, 10.3%, and 3.3% were underweight, average weight, overweight, and obese, respectively. The FM showed no significant change from the second to third trimesters in the underweight, overweight, and obese groups. Moreover, the FM in the overweight and obese groups did not change during any period. The FFM, TBW, and MM significantly increased from the first to second and second to third trimesters. In BMI-stratified multivariate regression analyses, FFM in the normal and overweight groups was positively associated with birth weight, whereas FM gain was negatively associated in the underweight and normal groups. No significant associations were observed in the obese group.

Changes in maternal body composition during pregnancy in Japanese women varied by pre-pregnancy BMI. Associations with birth weight also differed by BMI group. Further prospective studies are needed to confirm these relationships and investigate the mechanisms.