title: Treatment effects of maternal micronutrient supplementation vary by percentiles of the birth weight distribution in rural Nepal.
abstract: 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.
authors.affiliation: Department of International Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205-2103, USA. jkatz@jhsph.edu
authors.forename: Joanne
authors.initials: J
authors.lastname: Katz
authors.forename: Parul
authors.initials: P
authors.lastname: Christian
authors.forename: Francesca
authors.initials: F
authors.lastname: Dominici
authors.forename: Scott L
authors.initials: SL
authors.lastname: Zeger
mesh_terms: D000293:Adolescent; D000328:Adult; D001724:Birth Weight; D001827:Body Height; D001835:Body Weight; D002648:Child; D019587:Dietary Supplements; D005260:Female; D005865:Gestational Age; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008423:Maternal Age; D018977:Micronutrients / Q000494:pharmacology*; D009390:Nepal; D011247:Pregnancy; D011295:Prenatal Care; D012424:Rural Population; D015430:Weight Gain
chemical_list: D018977:Micronutrients