Multiple micronutrient supplementation during pregnancy does not lead to greater infant birth size than does iron-only supplementation: a randomized controlled trial in a semirural community in Mexico.

Randomized controlled trial in pregnant women from a semirural community near Cuernavaca, Morelos, Mexico, recruited before 13 weeks of gestation and followed through delivery. Birth outcome analyses were restricted to singleton live births in the intent-to-treat set, with 323 women in the multiple micronutrient group and 322 in the iron-only group.

Pregnant women in the active arm received a multiple micronutrient tablet containing vitamins A, D3, E, B1, B2, niacin, folic acid, B6, B12, C, zinc, iron, and magnesium. The supplement was taken orally 6 days per week from about 9 weeks of gestation until delivery; both groups also received 60 mg iron as ferrous sulfate, so the comparator was iron-only supplementation.

Multiple micronutrient supplementation did not improve infant birth size compared with iron-only supplementation. Adjusted differences were small and not significant for birth weight, 16.1 ± 31 g (P = 0.60), birth length, -0.002 ± 0.151 cm (P = 0.99), and ponderal index, 0.014 ± 0.018 kg/cm3 (P = 0.42). The adjusted odds ratios also showed no benefit for low birth weight, 0.98 (95% CI: 0.55, 1.74), intrauterine growth restriction, 0.82 (95% CI: 0.49, 1.39), intrauterine small size, 0.96 (95% CI: 0.59, 1.57), or preterm delivery, 1.16 (95% CI: 0.60, 2.23). Mean birth weight was 2.981 ± 0.391 kg in the multiple micronutrient group versus 2.977 ± 0.393 kg in the iron-only group, and mean birth length was 48.61 ± 1.82 cm versus 48.66 ± 1.83 cm.

There was substantial attrition before birth outcome analysis, with about one quarter of each arm lost to follow-up. Birth measurements were missing for some pregnancies, and the trial was conducted in a single semirural Mexican setting, which limits generalizability. The study also compared two active iron-containing regimens rather than supplement versus no supplement, which may reduce the ability to detect a modest incremental effect of multiple micronutrients.