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Which changes in maternal body composition influence birth weight?
  1. N Farah1,2,
  2. M Kennelly1,2,
  3. V Donnelly3,
  4. B Stuart1,2,
  5. MJ Turner1,2
  1. 1UCD Centre for Human Reproduction, Dublin, Ireland
  2. 2Coome Women and Infants' University Hospital, Dublin, Ireland
  3. 3Mount Carmel Hospital, Dublin, Ireland


Maternal weight gain during pregnancy is associated with increasing birth weight. It is uncertain how much fat mass (FM) and fat-free mass (FFM) contributes to this association. The authors examined the influence of different maternal body composition parameters on birth weight.

The study was confined to non-diabetic Caucasian women with a singleton pregnancy. Weight and height were measured in the first trimester. Maternal body composition was measured at 28 and 37 weeks using advanced bioelectrical impedance analysis. At delivery the baby was weighed and the clinical details recorded.

Of the 178 women, the mean body mass index (BMI) was 28.2 kg/m2. Birth weight did not correlate with early pregnancy BMI but did correlate with gestational weight gain (r=0.230; p=0.002). Maternal FM did not correlate with birth weight, however maternal FFM did (r=0.247; p<0.001). An increase in maternal FFM and not FM correlated with birth weight (r=0.469; p<0.001).

In non-diabetic pregnancies gestational weight gain, and not early pregnancy BMI, is the more important determinant of birth weight. Furthermore, it is the increase in maternal fat free mass and not fat mass that influences birth weight.

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