Objective: To evaluate the relationship between adenomyosis severity and perinatal outcomes. Design: Pregnant women with adenomyosis were divided into two groups: the mild group (adenomyosis volume <500 cm3) and the severe group (adenomyosis volume ≥500 cm3). Setting: Tertiary care institution, Shanghai, China. Population: In total, 286 pregnancies complicated with adenomyosis, who attended routine prenatal check-ups between July 1, 2017, and June 30, 2019, at Shanghai First Maternity and Infant Hospital participated in the study. Methods: A total of 286 pregnant women with adenomyosis were analysed retrospectively. The three diameter lines (length, L; width, W; and thickness, T) of adenomyosis were measured by ultrasound during pregnancy. The volume of adenomyosis (V) was calculated by the formula V= L *W * T * 4 / 3 π. Pregnant women with adenomyosis were divided into a mild group (V<500 cm3) and a severe group (V≥500 cm3). Main outcome measures: The relationship between adenomyosis severity and perinatal outcomes. Results: Pregnant women in the severe group had higher preterm labour rate (18.5% versus 10.2%;odd ratio 1.991;95%CI0.969-4.090) , higher caesarean section rate (67.9% vs 49.8%; odd ratio 2.136;95%CI1.114-2.672) , and earlier gestational week at delivery (38+0 ± 2+1 versus 38+5±2+0).The new-born birth weight was significantly lower(3056.7 ± 523.8g versus 3216.9 ±546.4g) in the severe group. Severe adenomyosis didn’t increase postpartum hemorrhage significantly. Conclusion: Severe adenomyosis is related to adverse perinatal outcomes.