Pregnancy is associated with an increase in body fat; however, excessive gestational weight gain predisposes to significant maternal and neonatal morbidity and mortality. It remains unclear whether alterations in energy homeostasis have a major influence on fat storage.

To evaluate longitudinal changes in body composition and energy metabolism in healthy pregnant women.

Body composition, energy expenditure and energy intake were measured longitudinally in 26 women with singleton pregnancies at 12–14 weeks, 24–26 weeks and 34–36 weeks of gestation. Fat mass (FM) and lean body mass (LBM) were measured using bio‐impedance analysis, total energy expenditure (TEE) using the Sensewear Armband and energy intake through a 3‐day food recall diary.

Throughout pregnancy, all women remained healthy. Body weight increased by 10.8 ± 3.9 kg, from 67.3 ± 14.1 kg to 78.1 ± 13.8 kg from the first to the third trimester (P < 0.001). Body fat percentage increased by 4.5 ± 4.2% (P < 0.001). LBM also increased throughout pregnancy, by 3.9 ± 2.4 kg (P < 0.001). TEE increased significantly from the first to the third trimesters (9514 kJ/day to 10 263 kJ/day; P < 0.05). In contrast, energy expenditure due to physical activity, energy intake and macronutrient intake did not change significantly throughout pregnancy.

Healthy women increase FM during pregnancy despite slight increases in TEE and no change in energy intake. This suggests that energy storage efficiency improves during pregnancy, which may be related to alterations in gut microbiota and activation of anabolic pathways during pregnancy. Clarifying factors leading to this more efficient fat and energy storing state, and the role of the pregnancy‐related changes in gut microbiota, may be important for managing gestational weight gain.