An allantoic cyst is a rare malformation with a frequency of 3 in 1,000,000 that may be seen antenatally by ultrasound assessment when the connection between the cloaca (future bladder) and the allantois fails to regress. A patent urachus that presents as a cyst (allantoic) is usually considered not to be associated with chromosomal abnormalities, but if it is not repaired after birth this leads to complications such as urinary tract infections and stone formation. We present a case of a fetus diagnosed with allantoic cyst at the first trimester ultrasound assessment at 12 weeks gestation. The follow up scans showed a decrease in size of the allantoic cyst with no other obvious major defects and, when invasive testing (amniocentesis with microarray analysis) was performed, a rare microdeletion, 1q21.1q21.2 was identified (1.82 Mb deletion).
