Pregnancy Due Date Calculator

An IVF pregnancy is the one case where the due date isn't really an estimate. Fertilisation happened in a lab, at a documented time, so there is no ovulation to guess at. The arithmetic simply subtracts the embryo's age at transfer from the standard 266-day post-fertilisation span:

• Day 3 transfer — transfer date + 263 days
• Day 5 blastocyst — transfer date + 261 days
• Day 6 blastocyst — transfer date + 260 days

A frozen embryo transfer is dated exactly the same way. Vitrification pauses development, so an embryo frozen on day 5 is still a day-5 embryo when it thaws, whether that was two months or two years later. Only the embryo's age at transfer matters, never the time it spent in storage — which is why one FET due date calculator works for fresh and frozen cycles alike.

Roughly 1 baby in 20 arrives on the estimated date. That sounds like a failure of the method until you realise the due date was never a prediction of one day — it's the midpoint of a distribution. Around 90% of babies arrive between 37 and 42 weeks, and week 39 is the single most common week to go into labour.

So the useful mental model isn't a deadline. It's a target month with a most-likely fortnight in the middle of it. Book the maternity leave, but don't plan anything unmissable for the week either side.

First babies are late more often than they are early, and passing 40 weeks is ordinary rather than ominous. What changes after that point isn't usually the pregnancy — it's the monitoring.

• 40–41 weeks. Still within the normal range. Many providers offer a membrane sweep around now and start talking through options.
• 41–42 weeks (late term). Monitoring usually steps up — non-stress tests and amniotic fluid checks, often twice weekly. Induction is normally discussed in this window.
• Beyond 42 weeks (post term). Uncommon, and most providers will have induced before this, because the risks of continuing begin to outweigh the risks of intervening.

Exactly when induction is offered varies by country, by hospital and by your own history. If your dates were revised earlier in pregnancy, it's worth confirming which due date the 41-week clock is being counted from — that single question resolves a lot of confusion at the end.

You conceived eight weeks ago and your notes say ten weeks. Both are right, and the gap is the explanation: gestational age is counted from the first day of your last period, which includes about two weeks before conception happened at all.

Medicine standardised on gestational age for a plain practical reason — almost everyone can name the date their last period started, and almost nobody can name the hour they conceived. Every scan report, blood test and growth chart you'll see uses it, so it's the number worth carrying in your head. The calculator above reports gestational age in weeks and days, the same way a scan report writes it.