Background Obstructive sleep apnea (OSA) is closely related to maternal obesity in pregnant women, and the association increases with later pregnancy. Obesity and OSA are risk factors of pregnancy-related complications, including gestational hypertension, gestational diabetes mellitus (GDM), and fetal morbidities. We aimed to determine the prevalence of OSA and to assess the impact of OSA on pregnancy-related disorders in overweight pregnant women. Methods Eligible participants who were overweight [body mass index (BMI) ≥ 23 kg/m 2 ] in gestational age 30 weeks or more, assessed OSA using a portable polysomnography at home. Clinical data were collected from pregnant women and their babies. Results The average age of 51 participants was 34.5 years (27–44 years). The number of primipara was 25 (49%) and that of multipara was 26 (51%). Eight cases of GDM (15.7%) and five cases of preeclampsia (9.8%) were reported, and six patients (11.8%) experienced preterm delivery. In results of polysomnography, 14 patients (27.5%) were diagnosed as OSA. Apnea-hypopnea index moderately correlated with BMI ( r = 0.515, P < 0.001). The BMI ( P < 0.005) and preeclampsia rate ( P < 0.017) were higher in the OSA group compared to the control group. Odds ratios (ORs) adjusting age, BMI, parity, and abortion history were calculated. The presence of OSA increased OR of preeclampsia (OR, 13.1; 95% confidence interval, 1.1–171.3). The majority of preeclampsia patients (4/5, 80%) underwent preterm delivery. Conclusion OSA is an important risk factor for preeclampsia, resulting in preterm delivery. For overweight pregnant women, an OSA evaluation should be mandatory.