Background: The Chilean Ministry of Health initiated powdered milk delivery to pregnant women in 1954. As a result of this and other health interventions, 99.9% of all pregnant women have regular ante natal checks and have their deliveries in maternity hospitals in Chile. The present powdered milk product delivered by the Ministry of Health to underweight pregnant women is called “Purita Fortificada” (PF). Women are classified as underweight using their weight/height ratio at the beginning of pregnancy. The Research Division of Parmalat spa and the Catholic University of Chile designed a new powdered milk-based product fortified with multi-micronutrients called Mamán (M). Amino-chelated iron is one of the novel components of this product. Omega-3 is also included. Aim: We report the results of an experimental trial of food supplementation in underweight Chilean women performed in 2002-2003. Fetal growth was compared in women receiving two different food supplements during pregnancy. Methods: Women were randomly assigned to either a control group receiving the regular 2 kg per month of powdered milk (PF) or an experimental group receiving 2 kg of M. The two groups were informed about the nutritional value and preparation of either supplement. Both are diluted at 10% and a daily consumption of 660 ml was recommended. During the monthly check-ups maternal weight changes and conditions or complications of pregnancy were registered. Women were also visited at home twice (20 and 35 w pregnancy) to study the home diet (24-h recall method) and socio-economic status. Results: Each group consisted of 332 (PF) and 347 (M) women. Maternal anthropometric variables at recruitment were similar between the two study groups (e.g. a BMI mean value of 20 in the two groups at week 11 of gestation). Other control variables such as the characteristics of the home diet and the family socio-economic status were also similar. Mean daily consumption of the two supplements was slightly higher in the control group (over 30 g/d in the two groups), i.e. somehow lower than two cups; the lower amount consumed in the experimental group is related to the presence of a higher proportion of non-consumers in this M group. Newborn characteristics in each study group were analysed according to the presence or absence of medical and obstetrical complications, including smoking. Smokers of at least one cigarette per day were 27 in the PF group meanwhile in the M group they were 33. The pregnancies of women in the experimental group lasted on average 273.3 ± 10.3 d, three days longer than the control group (p < 0,01). Mean birth weight was 124 g higher in the M group (p < 0.001) Proportions of low birth weight (< 2,501 g) and birth weight < 3,001 g were significantly lower in the experimental group. Significant differences in mean birth length and mean head circumference were also found. Maternal weight gain was somewhat higher in the experimental group but the difference was not statistically significant. Conclusion: Fetal growth in mildly malnourished women may be greatly enhanced with multi-micronutrients supplementation. These results are notably better than those previously published by us in the Santiago study (Mardones-Santander et. al.,1988). The Santiago study has been selected as one the four best trials done elsewhere (Pojda & Kelly, 2000). Mean birth weight in the M group almost doubled (124 g) the figure observed in the experimental group of the Santiago study (73 g) which also received a milk-based product fortified with multi-micronutrients. The present study also reached a favourable significant difference on birth length for the experimental group, a fact that is scarce in food-supplementation studies during pregnancy and is crucial for the intergenerational change of height during the nutrition transition as commented by Barker (1998). Short birth length has been found to be associated with adult mortality in Islandia (Gunnarsdottir et. al., 2002). These results are also consistent with previous data about the possible beneficial effects of omega-3 supplementation on pregnancy duration.