Pyelectasis is a collection of fluid in the fetal kidneys causing them to dilate. Pyelectasis is also referred to as "renal pelvic dilatation". It is a relatively common ultrasound finding, seen in approximately 1% of pregnancies, and is more common in male than in female fetuses. In the majority of cases, the fluid in the kidneys drains and does not have an impact on the baby's health.
Pyelectasis and Down Syndrome Risk
However, pyelectasis is considered an ultrasound "marker", which increases the chance that the baby may have Down syndrome. Although Down syndrome can occur in any pregnancy, the chance for Down syndrome increases with age. When pyelectasis is seen on ultrasound, the risk for Down syndrome is approximately one and one-half (1.5) times a woman's age-related risk.
Down syndrome is the most common type of chromosome abnormality in liveborn children. It is caused by an extra copy of chromosome #21. Chromosome abnormalities involving a missing or extra chromosome are usually not inherited and are not caused by an exposure during pregnancy. Instead, they are caused by a chance mistake in cell division at the time of conception. This error in cell division is a random event that can occur in any pregnancy. Children with Down syndrome have some degree of intellectual disability, characteristic facial features, and other health problems. Prenatal diagnosis by amniocentesis is available to test for chromosome abnormalities during pregnancy.
Pyelectasis and Other Health Risks
Babies with pyelectasis who are chromosomally normal (found not to have Down syndrome) have an increased risk for urologic problems that may require surgery after birth if the collection of fluid in the kidneys increases throughout pregnancy. If the pyelectasis persists, an ultrasound of the newborn kidneys is recommended. Typically, however, pyelectasis disappears by the third trimester of pregnancy and has no negative effect on the baby.
Genetic counseling to review this information in more detail is available.