How your due date is set, and why it matters
A first-trimester scan is more accurate than your dates.
Your due date determines almost everything that follows: when tests are offered, what counts as preterm, what counts as overdue, and when induction gets discussed. It is worth getting right.
An ultrasound in the first trimester dates a pregnancy more accurately than the date of your last period, because ovulation does not always happen when the calendar assumes. Once a first-trimester scan has set the date, it is not usually changed by later scans.
In randomized trials, routine first-trimester ultrasound compared with scanning only when there was a reason reduced short-term maternal anxiety (relative risk 0.80). Effects on pregnancy loss were too uncertain to draw a conclusion.
Write it downAsk what your due date is based on and whether it was set by scan or by dates. If someone later proposes induction "because you are 41 weeks," the accuracy of that number is the whole argument.
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.
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This page is educational. It supports the conversation with your own clinicians. It is not consent to any treatment, and it cannot assess you. If you are worried about a symptom now, see urgent warning signs.