Early ultrasound: what it can and cannot show
It confirms a great deal and rules out little.
An early scan confirms the pregnancy is in the uterus, whether there is one baby or more, whether there is a heartbeat, and how far along you are. In trials, routine second-trimester scanning detected multiple pregnancy far more reliably than not scanning (relative risk 0.05 for a multiple pregnancy going undetected).
What an early scan does not do is rule out most problems. In a study of more than a million pregnancies in England, only 32.7% of 14 major anomalies were detected before 16 weeks. Some conditions, such as anencephaly and abdominal wall defects, were found in more than 80% of cases that early. Others, such as diaphragmatic hernia and kidney agenesis, were largely missed.
Where this comes from:
141. Kaelin Agten A, Xia J, Servante JA, Thornton JG, Jones NW. Routine ultrasound for fetal assessment before 24 weeks' gestation. Cochrane Database Syst Rev. 2021;(8):CD014698.
144. Karim JN, Broughan JM, Aldridge N, Pandya P, McHugh A, Papageorghiou AT. Impact of first-trimester ultrasound on early detection of major fetal anomalies: nationwide population-based study of over 1 million pregnancies. PLoS Med. 2025;22:e1004709.
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