Bioimpedance analysis in a large cohort of women indicated an increase in fat mass during pregnancy, but the investigators did not quantify it due to the inaccuracy of bioimpedance analysis in estimating fat mass (24). Those studies where fat mass accumulation has been quantified using a variety of methods show a wide variation in the accumulation of fat during pregnancy of between 0 and >5 kg (5, 9-11, 23, 30-32). Studies following women from preconception until 15 weeks gestation observed that body fat had accumulated by the seventh week of gestation (33). Some studies indicate that the increase in maternal fat mass is complete by the end of the second trimester (23, 34) with a decline thereafter. Skinfold thickness, but not fat cell diameter correlated with these changes (34). Other studies indicate that most of the fat mass is deposited between 13 and 35 weeks of gestation (11). It has been suggested that during the third trimester, the fetus may require such a large proportion of energy that women do not deposit any additional fat (23). Fat gain during pregnancy may be required to provide sufficient energy for subsequent lactation, and women who have less than optimal energy intake during pregnancy cannot maintain bodyweight during lactation (35).
Baseline energy intake and change in energy intake during pregnancy were not correlated with the gestational fat gain during pregnancy. Prepregnancy weight, fat mass, and fat-free mass also did not predict the amount of fat deposited. There was some evidence that women with higher resting metabolic rates prepregnancy gained more fat (5, 23).
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