How we calculate your due date
A typical pregnancy lasts about 40 weeks (280 days) counted from the first day of your last menstrual period (LMP). This is Naegele's rule, the method most midwives and doctors start with:
- From your last period: due date = LMP + 280 days + (your cycle length - 28 days).
- From conception: due date = conception date + 266 days (38 weeks).
- From an IVF transfer: due date = transfer date + 261 days for a day-5 blastocyst, + 260 days for a day-6 blastocyst, or + 263 days for a day-3 embryo (that is, egg retrieval + 266 days).
- From a scan: we use the due date your scan gave you. If your report only shows how far along you were, due date = scan date + 280 days - that age (12 weeks 3 days is 87 days).
Your trimesters are counted from the same starting point: the first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Babies born from 37 weeks are "early term", from 39 weeks "full term", at 41 weeks "late term", and from 42 weeks "post-term" (ACOG definitions).
What to do next
Share your estimated date with your midwife or doctor at your first appointment. They may adjust it after your dating scan. Want to know how far along you are today? Try the pregnancy week calculator, or see how much weight gain is healthy for you.
