The authors examined the relationship of maternal anthropometry to fetal growth and birth weight among 1005 human immunodeficiency virus (HIV)-infected women in Lilongwe, Malawi, who consented to enrollment in the Breastfeeding, Antiretrovirals, and Nutrition Study (www.thebanstudy.org). Anthropometric assessments of mid-upper arm circumference (MUAC), arm muscle area (AMA), and arm fat area (AFA) were collected at the baseline visit between 12 and 30 weeks' gestation and in up to 4 follow-up prenatal visits. In longitudinal analysis, fundal height increased monotonically at an estimated rate of 0.92 cm/wk and was positively and negatively associated with AMA and AFA, respectively. These latter relationships varied over weeks of follow-up. Baseline MUAC, AMA, and AFA were positively associated with birth weight (MUAC: 31.84 g/cm(2), 95% confidence interval [CI], 22.18-41.49 [P < .01]; AMA: 6.88 g/cm(2), 95% CI, 2.51-11.26 [P < .01]; AFA: 6.97 g/cm(2), 95% CI, 3.53-10.41 [P < .01]). In addition, MUAC and AMA were both associated with decreased odds for low birth weight (LBW; <2500 g) (MUAC: odds ratio [OR] = 0.85, 95% CI, 0.77-0.94 [P < .01]; AMA: OR = 0.95, 95% CI, 0.91-0.99 [P < .05]). These findings support the use of MUAC as an efficient, cost-effective screening tool for LBW in HIV-infected women, as in HIV-uninfected women.