First-trimester megacystis is defined as enlargement of the fetal bladder, sonographically measured in the longitudinal diameter. It has been classified as being either mild (8–11 mm), moderate (12–15 mm) or severe (> 15 mm)1, 2. The concomitant finding of posterior urethral dilatation (‘keyhole’ sonographic appearance) or renal abnormalities (hydronephrosis and/or dysplastic kidneys) indicates, per se, a severe classification2. During the second and third trimester the diagnosis relies on subjective criteria2. Megacystis has been reported to be associated with various extrarenal anomalies3, megacystis–microcolon–intestinal hypoplasia syndrome4, 5 or various chromosomal aneuploidies1, 6-8. Multiple births are at higher risk for birth defects9. The risk ratio of bladder or urethra obstruction in twins over singletons is 3.34 (95% confidence intercal (CI), 1.20–9.27)9. Nevertheless, only two anecdotal cases describing megacystis in co-twins3, 5, have been reported. This motivated us to search our database of fetuses with severe abnormalities and to look for this condition among singletons and twins. Our inclusion criteria were diagnosis of a moderate to severely enlarged bladder1-3 and a gestational age of no more than 20 weeks (Figures 1 and 2)3. The current study cohort consisted of 335 consecutive pregnant women admitted to our department between January 1 1997 and April 31 2004, all of them because of severe fetal abnormalities. Three hundred and twenty were singleton pregnancies and 15 were dichorionic twin pregnancies. In every pair, only one of the fetuses had a severe abnormality (Figure 2). Karyotyping was performed in six fetuses and one case had trisomy 21 (Table 1)10. One fetus was miscarried, and the remaining affected pregnancies were interrupted following extensive counseling and upon the parents' request. In the twin pairs, selective feticide was performed and the unaffected fetus had an uneventful pregnancy outcome. Associated renal anomalies included obstructive common cloaca (Figure 1), echogenic kidneys and keyhole bladder (Table 1). When the case series' data were analyzed, it was found that megacystis was diagnosed in 7 (2%) of the singletons and in 3 (20%) of the twins from all the affected pregnancies, which represents a significantly higher rate among twin pregnancies than singletons (Table 2). Male fetus at 13 + 2 weeks' gestation showing a severely enlarged megacystis and a common cloaca (arrow) in (a) mid-sagittal and (b) transverse view. Twin pregnancy at 13 + 2 weeks' gestation. (a) Mid-sagittal section of twin showing the severely enlarged megacystis and the intertwin membrane (arrows); (b) mid-sagittal section of an apparently normal looking co-twin. Our data agree with those of previous reports3, 8 that fetal megacystis is a severe abnormality and the prognosis is dismal. This is mainly owing to a high rate of associated renal and extrarenal anomalies (5/9; 55% in our series). In agreement with Jouannic et al.3, we found no fetuses with increased nuchal translucency (NT) and we reiterate our previous recommendation to offer fetal karyotyping in cases of megacystis, irrespective of the NT or spontaneous resolution of this condition10. In agreement with Liao et al.8, it seems that megacystis is predominantly a male associated abnormality. The occurrence of this abnormality in a female fetus should alert the search for either megacystis–microcolon–intestinal hypoperistalsis or an abnormal karyotype. The incidence of discordance for birth defects in siblings from the same multiple pregnancies may be due to maternal hemodynamic or microvascular changes in the placenta, as well as nutritional supply. It has already been shown that even in genotypically identical twins, the incidence of birth defects may be discordant, pointing probably to a role for nongenetic factors in the etiology of birth defects9. We are aware that our current study is not an epidemiological one, but rather a small case series which may be subject to some bias. Nevertheless, it may prompt others to search their database for similar conditions. R. Maymon*, I. Ben-Ami*, Z. Vaknin*, O. Reish , A. Herman*, * Department of Obstetrics Gynecology, Assaf Harofeh Medical Center, Zerifin, 70300, Israel, The Genetic Institute, Assaf Harofeh Medical Center, Zerifin, affiliated with the Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel