Malnutrition during the critical period of pregnancy has significant health outcomes for both the mother and her offspring. Medical nutrition therapy (MNT) by a registered dietitian nutritionist (RDN) may help mitigate negative health effects, although studies that support the role of the RDN have not been comprehensively evaluated. The objective was to explore the health effects of MNT by an RDN on maternal and infant outcomes in pregnant women with malnutrition. A systematic review of studies published between 2000 and 2014 that incorporated MNT by an RDN during pregnancy were retrieved from a PubMed search, using criteria established by the Academy of Nutrition and Dietetics Evidence Analysis Process. Among 94 identified studies, five controlled trials met the inclusion criteria. The initial search was extended to include one study published between 2014 and 2019. Outcomes included maternal gestational weight gain, maternal markers of glycemic control, maternal complications such as hypertension, incidence of caesarean section, infant birth weight both in grams and in clinical categories, infant gestational age, and infant complications. There was good/strong evidence that MNT by an RDN decreased gestational weight gain, although there was no effect on maternal complications, caesarean section deliveries, and gestational age among women with mixed body mass index status or those who were overweight/obese. The evidence was deemed fair in support of an effect on glycemic control, infant birth weight, and infant complications. The heterogeneity in the results are due to the variation among populations studied, types of interventions, and inconsistency among outcomes. In addition, the training and educational requirements of the RDN or the international equivalent may vary widely across the four countries in which studies were conducted. There was good evidence for MNT by an RDN during pregnancy on improving gestational weight gain among overweight/obese women. To better support the role of MNT by an RDN in the health care of pregnant women, research that clearly identifies the role of the RDN in the intervention, includes a control group, and studies more heterogeneous populations is needed.