This study was a randomized, double-blind clinical trial in semirural Mexico to compare the effects of multiple micronutrient supplements (MMS) with those of iron supplements during pregnancy on birth size. Pregnant women received supplements, 6 days a week at home, as well as routine antenatal care, until delivery. Both supplements contained Iron but the MMS group also received 1–1.5 times the recommended dietary allowances of several micronutrients. Both groups did not differ significantly in terms of the birth size of newborns. The results suggest that MMS during pregnancy does not lead to greater infant birth size than does iron-only supplementation.