OBJECTIVE Although a higher maternal body mass index is associated with preterm birth, it is unclear whether excess gestational weight gain (GWG) or obesity drives increased risk. We and others have shown that the placenta harbors microbiota, which is significantly different among preterm births. Our aim in this study was to investigate whether the preterm placental microbiome varies by virtue of obesity or alternately by excess GWG. STUDY DESIGN Placentas (n = 320) were collected from term and preterm pregnancies. Genomic DNA was extracted and subjected to metagenomic sequencing. Data were analyzed by clinical covariates that included the 2009 Institute of Medicine’s GWG guideline and obesity. RESULTS Analysis of 16S recombinant RNA–based metagenomics revealed no clustering of the microbiome by virtue of obesity (P = .161). Among women who spontaneously delivered preterm, there was again no clustering by obesity (P = .480), but there was significant clustering by excess GWG (P = .022). Moreover, among pretermbirths, detailed analysis identified microbial genera (family and genus level) and bacterial metabolic gene pathways that varied among pregnancies with excess GWG. Notably, excess GWG was associated with decreased microbial folate biosynthesis pathways and decreased butanoate metabolism (linear discriminate analysis, >3.0-fold). CONCLUSION Although there were no significant alterations in the microbiome by virtue of obesity per se, excess GWG was associated with an altered microbiome and its metabolic profile among those women who experienced a preterm birth.

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The incidence of obesity in pregnancy has increased over the past two decades, with nearly 50% of U.S. women aged 15–49 are classified as overweight or obese. Obesity (independent of diabetes) among gravidae poses unique risks which extend towards the fetus, with several large population-based analyses demonstrating independent increased risks for fetal malformations including neural tube defects, cardiac anomalies, and orofacial clefts as well as stillbirth and macrosomia. Unfortunately, several lines of evidence also suggest that the quality of the prenatal fetal anatomic survey and certain aspects of prenatal diagnostic screening programs are significantly limited. The net effect is that among obese gravidae, the increased risk of fetal anomalies is further offset by a concomitant diminished ability to sonographically detect such malformation in the prenatal interval. The purpose of this summary review is to systematically examine the evidence suggesting an increased risk of fetal malformations in obese gravidae, the contributing role of diabetes, and the limitations of prenatal diagnostic and sonographic screening among this at-risk population.