Background: The microbiota-dependent metabolite trimethylamine-N-oxide (TMAO) has been reported as a novel and independent risk factor for the development of cardiovascular and metabolic diseases, but the association with gestational diabetes mellitus (GDM) remains unclear. Objective: To investigate the association between plasma TMAO concentration and GDM in a two-phase study. Design: The initial discovery phase included 866 pregnant women (433 GDM cases and 433 matched controls) in Wuhan China. Study participants were recruited from pregnant women who attended the outpatient clinics of the Department of Endocrinology, Tongji Hospital, to screen for GDM between August 2012 and April 2015, or pregnant women who visited the Hubei Maternal and Child Health Hospital or the Central Hospital of Wuhan for a routine antenatal checkup from May 2014 to November 2016. The inclusion criteria of participants were: age ≥ 20 years, gestational age between 24 and 32 weeks, no history of a diagnosis of diabetes or gestational diabetes, and no history of receiving pharmacological treatment known to affect glucose metabolism. An independent replication phase study was nested within an ongoing prospective cohort study, namely the Tongji Maternal and Child Health Cohort (TMCHC). Beginning in January 2013, women receiving prenatal care prior to 16 weeks of gestation were invited to join the TMCHC. Exclusion criteria included pre-pregnancy diabetes, clinically significant neurological, endocrinological or other systemic diseases and multiple pregnancies. All enrolled pregnant women received a regular prenatal checkup in hospital and underwent an oral glucose tolerance test (OGTT) during 24-32 weeks of gestation to screen for GDM. 276 members who developed GDM before May 2016 and had fasting blood collected before 20 weeks'gestation were included as cases in this analysis. Two controls were individually matched to each case from among women without GDM. These two studies were approved by the ethics committee of Tongji Medical College. All the participants gave informed written consent before they were included in the study. Plasma TMAO concentrations were determined by stable isotope dilution liquid chromatography with online electrospray ionization tandem mass spectrometry.