A full-term pregnancy runs anywhere from 37 to 42 weeks, and the standard way to estimate a due date is to count 280 days from the first day of your last period. That’s what this calculator does, whether you know the date of your last period, the date you conceived, or the date of an IVF transfer.
It’s worth saying upfront: this is an estimate. Only around 1 in 20 babies arrive on their exact due date. Your dating scan at around 12 weeks will usually give a more accurate figure, based on your baby’s measurements rather than your cycle.
How the calculator works out your due date
There are three common starting points, and this tool lets you use whichever one applies to you.
From your last menstrual period (LMP): the most widely used method. Pregnancy is dated from the first day of your last period, even though conception typically happens around two weeks later. Add 280 days (40 weeks) to get your estimated due date. This is also the method the NHS uses by default.
From your conception date: if you know roughly when you conceived, perhaps from ovulation tracking, add 266 days (38 weeks) instead.
From an IVF transfer date: for a day-5 blastocyst transfer, add 261 days. For a day-3 transfer, add 263 days. IVF dating tends to be more precise than LMP-based estimates, since the exact age of the embryo at transfer is known.
Cycle length matters too. The calculator adjusts for cycles longer or shorter than the average 28 days, since ovulation, and therefore conception, shifts along with it.
What happens after you get your estimated date
Your first port of call is your GP or midwifery team, who will book you in for a booking appointment, usually between 8 and 10 weeks. They’ll go through your medical history, talk you through your options, and get your dating scan booked.
The dating scan, at around 11 to 14 weeks, measures your baby from crown to rump and gives a more accurate due date than the calculation above. If the scan date differs from your calculated date by more than a few days, the scan date is generally the one your care team will use going forward.
Trimester by trimester: what to expect
First trimester (weeks 1-12): early pregnancy symptoms, your booking appointment, and your dating scan.
Second trimester (weeks 13-26): often called the most comfortable stretch. Your 20-week anomaly scan happens here, and many parents start thinking seriously about the nursery.
Third trimester (weeks 27-40+): final growth scans if needed, your maternity leave planning, and getting practical items- cot, car seat, changing space- ready to go.
When to start getting nursery-ready
There’s no fixed rule, but most parents begin sorting the nursery from around 28 weeks, leaving a comfortable buffer before their due date without the pressure of doing it all in the final fortnight.
A bedside crib or Moses basket covers the newborn stage, with most families moving to a full-size cot bed somewhere between 4 and 6 months. Whichever you choose, always use a new mattress. The Lullaby Trust is clear that a used mattress carries a higher SIDS risk, even from a cot that’s otherwise in good condition.
Conclusion
Whichever method you use, your due date is a working estimate rather than a fixed appointment, and your dating scan is where that estimate gets refined. In the meantime, it’s a useful marker for planning: booking appointments, maternity leave, and working out when to start on the nursery furniture and storage.
