Background and aim There is limited knowledge about the effect of maternal metabolic syndrome (MetS) on the anthropometric parameters of newborns. Therefore, the authors aimed to evaluate the association between MetS in the first trimester of pregnancy with weight and height of the newborn. Methods This prospective cohort study was conducted on 455 pregnant women in Tehran during their first trimester of pregnancy. MetS was defined as the coexistence of three or more of the following criteria: fasting blood sugar (FBS) level ≥92 mg/dl, blood pressure ≥130.85 mm/hg, triglyceride ≥150 mg/dl, high density lipoprotein ≤50 mg/dl, and body mass index (BMI) ≥30 kg/m2. All participants were followed up to childbirth. After birth, the baby's weight and height data were collected from the birth certificate. Results Linear regression analysis showed FBS (ß: 0.100, p-value: 0.038), BMI (ß: 0.139, p-value: 0.004), and MetS (ß: -0.122, p-value: 0.015) were significantly associated with birth weight but no statistically significant results were found for birth height. Conclusion MetS and some of its components in pregnancy can affect birth weight of neonates.