Changes in the gut bacteria during pregnancy and a lack of vitamin D can have negative health consequences for both mother and child. Vitamin D has important functions in the human body and is key in regulating immune and inflammatory responses. Evidence suggests that vitamin D helps to maintain gut wall integrity and regulate inflammatory mechanisms in response to bacteria. Gut bacteria themselves have immune regulatory functions, and unfavourable disruptions in the composition of the bacteria are associated with chronic inflammatory diseases. Evidence shows gut bacteria composition is influenced by Vitamin D. During pregnancy, a substantial alteration in gut bacteria composition occurs and as pregnancy advances, there's typically an increase in bacteria linked to inflammation. This study analysed the data from the Vitamin D Antenatal Asthma Reduction Trial (VDAART, 2014), a randomised placebo controlled trial which gathered information on the impact of vitamin D on various markers as well as gut microbiome composition in pregnant women. For the study all participants took a daily multivitamin with 400 IU Vitamin D during the third trimester of pregnancy, and were given either an additional 4000IU of Vitamin D or a placebo. Results were drawn from 114 participants and their baseline vitamin D levels in early pregnancy, its changes over the trial period, as well as gut bacteria composition. The vitamin D levels at the start aligned with expected outcomes and was strongly linked to race, income, and education level. The baseline vitamin D level in early pregnancy also showed a connection to certain gut microbiome composition. However, these bacterial constellations remained robust and did not show any changes in response to Vitamin D supplementation throughout pregnancy. During the trial, most participant's vitamin D levels increased, especially those in the 4400 IU treatment group. Interestingly, women whose vitamin D levels did not increase much throughout the trial displayed a higher abundance of a bacteria called Desulfovibrio. Desulfovibrio is associated with an increased incidence of respiratory and inflammatory bowel diseases and the authors suggested that increasing vitamin D during pregnancy might help prevent the growth of more unfavourable bacteria like Desulfovibrio. Further long-term research is needed to confirm this idea.