To determine the imaging features and clinical value of first-trimester ultrasonographic screening of fetal megacystis in multifetal pregnancy.

Retrospective analysis was undertaken in 2159 cases of multifetal pregnancy screened in our hospital at gestational age from 11th to 13^{+ 6}th weeks by color Doppler ultrasound between Jan 2011 and April 2016. Fetal bladder was defined by showing two umbilical arteries and the longitudinal length of the bladder was measured in mid-sagittal plane. Clinical progression of fetal megacystis diagnosed in first-trimester were followed and the outcomes were recorded.

Totally six cases of fetal megacystis in multifetal pregnancy were detected, in which five cases were detected in one fetus of twin pregnancy and the other case was detected in one fetus of triplet pregnancy. In all cases of fetal megacystis, longitudinal length of the fetal bladder was more than 7 mm and double umbilical arteries were detected. In two cases of naturally-occurring twin pregnancy, abortion was performed and postnatal autopsy revealed prune belly syndrome. In the rest four cases of multifetal pregnancy through in vitro fertilization-embryo transfer (IVF-ET), two cases of twin pregnancies underwent selective feticide. The other two cases of twin and triplet pregnancies refused intervention, among which still birth happened in the megacystis fetus and spontaneous fetal death at gestational age of 13^{+ 2} weeks happened in the monochorionic monoamniotic twins of the triplet pregnancy. Totally, four live babies were born, among which three were healthy. The other baby was found to have 1.5 Mb deletion in chromosome 17q12 by microarray at gestational age of 32 weeks due to increased renal echogenicity.

First-trimester ultrasonography can effectively detect and diagnose megacystis in multifetal pregnancy. Early diagnosis and timely intervention are helpful for improving the outcome of fetal megacystis in multifetal pregnancy.