During the study period, 24 492 pregnant women attended the Harris Birthright Research Centre at 10–14 weeks of gestation, at which time, in addition to the measurements of nuchal translucency thickness and crown-rump length (CRL), data on fetal abnormalities were recorded onto a computer database. Cases of megacystis were identified and the records were reviewed. Additionally, the relationship of the longitudinal bladder diameter with the CRL and the bladder diameter/CRL ratio (expressed as a percentage) were examined with the use of data from 300 normal fetuses at 10–14 weeks.
Megacystis was present in 15 of the 24 492 pregnancies (1 in 1633) and in these cases the minimum longitudinal bladder diameter was 8 mm and the minimum bladder diameter/CRL ratio was 13 %. In the 300 control fetuses the bladder was visualized in 278 (92.7%) of the cases and the longitudinal bladder diameter increased with the CRL (bladder diameter = 0.065 × CRL − 0.69; r = 0.47, p < O.OOl), none of the measurements was more than 6 mm and the median bladder diameter/CRL ratio was 5.4% (range 0–l 0.4%) which did not change significantly with gestation (r = 0.1, p = 0.09). The bladder was visible in all cases with a minimum CRL of 67 mm. In three of the IS cases with megacystis, there were chromosomal abnormalities. In the chromosomally normal group, there were seven cases with spontaneous resolution, whereas in four cases there was progression to severe obstructive uropathy.
The bladder diameter was 8–12 mm and the bladder diameter/CRL ratio 13–22% in all cases with resolution and in one case with progressive megacystis; in the other three cases with progressive obstruction, the bladder length was more than 16 mm and the bladder diameter/CRL ratio was more than 28%. Copyright © 1996 International Society of Ultrasound in Obstetrics and Gynecology