Pregnant woman 31 years old, 30 weeks pregnant, 2nd child. The first child was healthy and denied any history of illness and exposure to toxic and harmful substances during this pregnancy. The ultrasound report concluded no abnormality around 20 weeks of pregnancy, but the ultrasound examination at 30 weeks of pregnancy revealed a fluid mass in the fetal pelvis, and the local hospital suspected anal atresia and came to our hospital for review. Ultrasound examination: fetal biparietal diameter 8.0cm, head position, 7.0cm long diameter, 3.2cm short diameter, 3.9cm transverse diameter liquid mass in fetal pelvis, the rest of the limbs, spine, chest, heart, lungs and abdominal organs, placenta, umbilical cord blood flow, amniotic fluid are not abnormal, no intestinal dilatation is seen. The mass was a cone-like shape with the tip pointing to the perineum, regular shape, smooth inner wall, and located behind the bladder (Figure 1); the wall of the mass was thicker, the tip was 6.2 mm thicker than the body, and the central part had a lip-like projection in the anechoic zone, which was neat, about 5 mm long and 11 mm wide, with wired-like structures around the periphery and in the middle, and the anechoic zone was poorly permeable and uniform fine dotted echogenicity was seen (Figure 2). Ultrasound diagnosis: fetal developmental abnormalities pelvic fluid mass, genital developmental malformations were considered and she was instructed to review postpartum. Postpartum examination of the baby: anus and external urethral orifice were present, no external vaginal orifice, locally visible blunt tip-like elevation, palpable and pushable into the pelvis, with pressure sensation; abdominal ultrasonography: pelvic fluid mass shape and echogenicity were similar to prenatal examination, 3 diameters were 7.9 cm, 4.4, 3.7 cm, changes in wall thickness were not obvious, apical elevation was more obvious, significantly larger than prenatal, elevated structure and fluid mass The apical lateral mushroom-shaped parenchymal echogenic mass continued, and the line of separation visible in the middle also continued to its middle (Figure 3); the lower end of the anechoic area widened compared with the prenatal period and gradually narrowed into a blind end passing through the pelvic floor, forming a bulge between the urethral orifice and anus of the perineum in appearance (Figure 4). Clinically, an incision of the vulvar bulge was performed, and a darker and turbid, thin fluid flowed, with a large number of epithelial cells detected microscopically, and the fluid mass in the pelvis disappeared. Final diagnosis: fetal vaginal effusion. Discussion Vaginal effusion is a disease caused by atresia of the hymen or external vaginal opening and is a kind of fetal genital developmental malformation. The fetal uterine glands are subjected to maternal estrogen and secrete a large amount of mucus to accumulate in the vagina to form a cystic mass, and the uterus is usually not visible or not dilated; the bladder and both kidneys may also be obstructed, and a cloaca manifesting as a joint bladder and vagina may appear, and this case belongs to the former. Analysis of ultrasound diagnostic points: 1. Location Located posterior to the bladder, without bowel dilatation, female fetus. 2. Structure of the mass The wall of the mass is relatively thick, which basically excludes the possibility of simple cyst; morphologically there is a certain duct-like shape, not round, and the wall is smooth, which can exclude fluid teratoma and meconium cyst; the top of the mass is raised with regular structure and centered, which should be considered as the possibility of ectocervix. However, when the uterus is normally developed, excessive fluid accumulation can cause co-dilation of the uterus.