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Hydronephrosis in newborns (a swollen kidney in a baby): the causes, the tests, and the treatment
For anyone looking after a baby, or expecting one, who has been told a kidney is swollen or dilated: on a pregnancy ultrasound, or after the birth. It passes on, by name, what the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, part of the NIH) says: what hydronephrosis is and how common it is, when it can be a sign of a problem, the signs in a newborn, what causes it, how it is found before and after birth, and the treatment (watchful waiting, preventing infections, and surgery). It builds the words to say to your baby’s doctor or nurse, so you do not have to find them, and prints a card.
Right now
- Your baby seems unwell, has a fever, or you are frightened? If you believe it is an emergency, call 911 (ours; NIDDK’s page does not name 911). For a fever, call your baby’s health care professional now: Fever · A child’s urinary tract infection.
- Often it is mild and goes away on its own: in about half of the pregnancies where it is found, it is gone by birth (NIDDK).
- Is it all too much, or are you thinking of ending your life? Call or text 988, any hour (ours).
More help
- On this site: Kidneys and bladder: find the right page · A child’s health: find the right page · Kidney disease in children.
- If your baby is unwell: Fever · A child’s urinary tract infection.
- Getting ready: Getting ready for urinary tract imaging · Get ready for your appointment · My medicines · When you need a doctor and can’t pay.
- For you: Pregnancy and a new baby · Depression after a baby.
- In a crisis: Help right now (988, 911). All our free tools.
This page and its wording were written with AI (Claude) for Scarlet Beast (our AI policy). No doctor or nurse has reviewed it. The facts were read on October 18, 2026 on NIDDK’s (NIH) page Hydronephrosis in Newborns (last reviewed October 2019) and its Spanish page Hidronefrosis en recién nacidos (last reviewed October 2019), which say the same. The numbers (1 or 2 out of every 100 pregnancies; about half gone by birth; males about twice as likely; usually gone by age 3 when it is transient) are NIDDK’s. Ours: when to call 911 (NIDDK’s page does not name it); calling about a fever; 988; that nothing on NIDDK’s page names anything a parent did; the questions to ask; the links to our other pages; and the order of the steps. What we left out: the four references NIDDK cites, the drawings, the section on clinical trials, NIDDK’s outside links, and its separate page on vesicoureteral reflux. This page saves nothing and loads no trackers.