Health Hub · Free tool
Due Date Calculator
Find out when your baby is due and how many weeks pregnant you are right now — from the first day of your last period, the same starting point your midwife will use.
Your dating scan gives the medically official due date — if it differs from this estimate, the scan wins.
How the due date is calculated
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception — which is why "40 weeks pregnant" includes roughly two weeks before conception even happened. The calculator uses the standard method (Naegele's rule): LMP plus 280 days, adjusted for your cycle length, since later ovulation in longer cycles shifts everything along. It's the same arithmetic on the wheel your midwife uses at booking.
How many weeks pregnant am I?
The calculator counts complete weeks from your LMP — so "8 weeks and 3 days" means you're in your ninth week. Weeks 1–12 are the first trimester, 13–27 the second, 28 to birth the third. If you conceived through IVF or know your exact conception date, dating runs from that instead (conception plus 266 days) — mention it at booking.
How accurate are due dates, honestly?
Only about 1 in 20 babies arrives on the due date. It's the centre of a distribution, not an appointment: most babies come within a fortnight either side, and "term" spans weeks 37 to 42. The dating scan at 11–14 weeks measures the baby directly and replaces the calendar estimate — from that point on, the scan date is the official one.
What to do next
- Refer yourself to a midwife now — in most of the UK you self-refer online to your local maternity service; no GP appointment needed. Booking happens around weeks 8–10.
- Folic acid 400mcg daily until week 12, and vitamin D 10mcg throughout — both in our pregnancy vitamins range.
- Check your medicines. Everyday remedies — ibuprofen among them — aren't all pregnancy-safe. Run anything you take past our pharmacist or your midwife.
- Not certain you're pregnant yet? Our pregnancy tests are reliable from the first day of a missed period.
Frequently asked questions
Can my due date change?
Yes — the dating scan commonly moves it by a few days, occasionally more if cycles were long or irregular. After the scan it stays fixed.
I don't know my last period date — what now?
Give your best estimate and let the dating scan do the work; it's accurate to within about five days in the first trimester regardless of period dates.
When should I tell my GP or midwife?
As soon as you know — early booking means earlier screening choices, and it's the safest path if you take regular medicines or have ongoing conditions.