Background: Birth weight is of importance due to its relation to fetal health, and it's also a predictor of the subsequent development of the child., Objective: This study aims to assess whether betaine is associated with poor fetal growth., Design: A case-control study was used in this study., Setting: The study took place at the Chongqing Maternal and Child Health Hospital, in Chong Qing, China., Participants: A total of 141mother-infant pairs were recruited from the Department of Obstetrics of our hospital between June 2021 and December 2021. According to gestational age and birth weight, themother-infant pairs were divided into small-for-gestational-age and appropriate-for-gestational-age groups., Primary Outcome Measures: Cord plasma concentrations of betaine were measured by high-performance liquid chromatography tandem mass spectrometry. Plasma levels of triglycerides, low-density lipoprotein, high-density lipoprotein and total cholesterol were determined using commercially available assays on an automatic biochemical analyzer (BS-240 VET, Mindray Medical, Shenzhen, China) using reagent from Mindray Medical company., Results: Cord plasma betaine concentrations were higher in small-for-gestational-age relative to appropriate-for-gestational-age newborns, and were not correlated to lipid levels. Adjusting for maternal and neonatal characteristics, birth weight and birth length were negatively correlated with the levels of betaine. Higher betaine concentrations were associated with increased risks of small-for-gestational-age., Conclusions: Elevated cord blood betaine concentration was independently associated with a higher risk of small-for-gestational-age infants, suggesting that betaine dysregulation may be a risk factor for impaired fetal growth.