A vesicocentesis involves using a needle to withdrawal excess fluid from the fetal bladder. This is done under ultrasound guidance. Fetal urinary tract obstruction is a condition that is caused by the blockage of fetal urination. Because your baby cannot empty its bladder in utero, the bladder may become very large and distended resulting in the need for drainage. This procedure may also be used to assess the character of fluid in the bladder.