OBJECTIVE:To assess anti-Müllerian hormone (AMH) levels as a marker of early abortion in the first trimester among women younger than 35 years. METHODS:Prospective study of women aged 18-34 years with a spontaneous pregnancy at less than 12.6 gestational weeks in Łódź, Poland, between January 2017 and November 2018. Cases of anembryonic abortion and assisted conception were excluded. Blood samples were collected and assayed for serum AMH levels. Data were compared between women with an embryo with no cardiac activity by ultrasound (n=30) and those with a normal embryo (n=33) by using Statistica12 software. RESULTS:The 10th and 90th percentiles of serum AMH concentration among control women were used to identify a reference AMH range (1.1-4.5 ng/mL). Risk of pregnancy loss in the first trimester was found to be higher for both low AMH (<1.1 ng/mL; relative risk [RR], 3.66; 95% confidence interval [CI], 2.1-6.4; P<0.001) and high AMH (>4.5 ng/mL; RR, 3.0; 95% CI, 1.61-5.59; P<0.001). CONCLUSIONS:Both very low and very high AMH concentrations were found to significantly increase the risk of abortion in the first trimester of pregnancy. Serum AMH might be a valuable marker to predict the risk of early abortion.