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Community and International Nutrition

Treatment Effects of Maternal Micronutrient Supplementation Vary by Percentiles of the Birth Weight Distribution in Rural Nepal^1,2

Joanne Katz^*,,3, Parul Christian^*, Francesca Dominici and Scott L. Zeger

^* Department of International Health, Department of Biostatistics, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205-2103

³ To whom correspondence should be addressed. E-mail: jkatz{at}jhsph.edu.

Certain antenatal micronutrient supplements increased birth weight by 40–70 g in rural Nepal. The effect was estimated by calculating the mean difference in birth weight between control and treatment groups, which assumes a constant treatment effect across the birth weight distribution. By estimating differences (and CI) in birth weight between treatment and control groups as a nonlinear, smooth function of the percentiles of the birth weight distribution, we can examine whether the shape of the birth weight distribution for a treatment group is different from that of the control group. Supplementation groups were folic acid, folic acid and iron, folic acid and iron and zinc, and a multiple micronutrient supplement all with vitamin A, compared with the control group of vitamin A alone. The shape of the birth weight distribution in the multiple micronutrient group was the same as that of the control group; however, the location of the distribution had shifted. The folic acid and iron group had fewer infants in the lower tail of its distribution but a similar proportion in the upper tail compared with the control group. The biologic pathways affecting intrauterine growth may vary by micronutrients such that some may confer a benefit among the most vulnerable infants, whereas others may have a more constant effect across the birth weight distribution. Future analytic approaches to estimating benefits of maternal supplementation on birth weight should examine whether there is a constant or variable treatment effect across the distribution of birth weight.

KEY WORDS: • birth weight • micronutrients • pregnancy • infant mortality • Nepal

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