The relationship between changes in maternal subcutaneous fat and infant birth weight was studied in 608 low income women. A loss of upper arm fat area (> 6.4 cm2), measured from 28 wk gestation to 4-6 wk postpartum, was associated with greater birth weight (+144 g, P < 0.01). However, when pregravid weight was low (< 25th percentile for age), a loss of upper arm fat area was associated with a birth weight lower by more than 300 g than that for women with higher pregravid weights who also lost fat, indicating that maternal stores among those with low weight may have been relatively depleted. Continued gains in upper arm fat area (> 5 cm2) from 28 wk gestation to the postpartum period was also associated with a lower birth weight (-123 g, P < 0.02). The mothers who gained upper arm fat late in pregnancy or continued to accrue fat in the postpartum period had the largest gestational weight gains, bore infants who were smaller, and retained the most weight postpartum. Thus, change in upper arm fat is a significant predictor of variation in infant birth weight.