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Semin Perinatol. 2013 Oct;37(5):307-9. doi: 10.1053/j.semperi.2013.06.006.

Should a first trimester dating scan be routine for all pregnancies?

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Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, 75 Francis St., OBC-3-010, Boston, MA 02115. Electronic address:


Accurate pregnancy dating is important for diagnostic and management decisions during pregnancy. Dating by last menstrual period is less reliable than dating by ultrasound, since the former has memory-related and other sources of inaccuracy. Dating by first trimester ultrasound, via the crown-rump length, has a 95% confidence range of ±5 days, slightly better than the ±8 days for dating by second trimester ultrasound at 14-20 weeks. This small difference appears to be too small to justify a routine first trimester scan for the purpose of dating the pregnancy.


First trimester; Gestational age; Last menstrual period; Second trimester; Ultrasound

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