Early diagnosis of fetal defects

The fetal bladder can be visualized by sonography in about 95% of fetuses at 11 weeks of gestation and in all cases by 13 weeks. At this gestation the fetal bladder length is normally less than 6 mm. The fetal kidneys are visible in about 75%, 85% and 95% of cases at 11, 12 and 13 weeks, respectively.

• Fetal megacystis in the first-trimester, defined by a longitudinal bladder diameter of 7 mm or more, is found in about 1 in 1,500 pregnancies.
• When the longitudinal bladder diameter is 7-15 mm the incidence of chromosomal defects, mainly trisomies 13 and 18, is about 20%, but in the chromosomally normal group there is spontaneous resolution of the megacystis in about 90% of cases.
• In contrast, in megacystis with bladder diameter greater than 15 mm the incidence of chromosomal defects is about 10% and in the chromosomally normal group the condition is invariably associated with progressive obstructive uropathy leading to hydronephrosis and dysplastic kidneys.
• Megacystis is associated with increased NT, which is observed in about 75% of those with chromosomal defects and in about 30% of those with normal karyotype.