Estimate your due date, track trimester progress, and see key pregnancy milestones week by week.
⚕️ Disclaimer: This calculator provides estimates only. Only about 4% of babies arrive on their exact due date. Consult your OB-GYN for accurate pregnancy dating via ultrasound.
The Due Date Calculator estimates when your baby is due and tracks trimester milestones from your last menstrual period or conception date, so you can count down the weeks with confidence.
Naegele's rule adds 280 days to the first day of your last period. Which is odd on the face of it, because you weren't pregnant then — conception happens around two weeks later.
The convention survives because it's anchored to something observable. Almost nobody knows their conception date; nearly everybody knows when their last period started. So obstetrics standardised on the date you can actually report and absorbed the two-week offset into the numbering. It's a measurement convention, not a biological claim.
This also explains the thing that confuses every first-time parent: you take a test, it's positive, and you're immediately "four weeks pregnant". Four weeks since your last period; about two since conception. This tool reports gestational age the way a midwife will — completed weeks since your last period — precisely so the number on your screen matches the number in your notes.
If you know your conception date instead, the calculator works backwards 14 days to the equivalent starting point. For IVF it adjusts for the transfer day as well, since a day-5 blastocyst is already five days into development when transferred — which is how clinics date IVF pregnancies.
Less than the precision suggests, and that's worth knowing early. Naegele's rule assumes a 28-day cycle ovulating on day 14. If your cycles run longer or shorter, your true date shifts by roughly the difference — a 30-day cycle puts it about two days later.
Only about 4% of babies arrive on their estimated date, and full term is anything from 37 to 42 weeks. A first-trimester dating scan is more accurate than any date arithmetic, and where the two disagree, the scan wins. Treat the output as the centre of a five-week window rather than an appointment — it will save you a fortnight of feeling overdue.
Trimesters are marked at the end of week 12 and the end of week 27, the standard split. In practice the dates people care about are the ones that attach to appointments: the dating scan around weeks 11 to 14, the anomaly scan around 18 to 21, viability thresholds in the mid-twenties, and full term at 37.
Having these as calendar dates rather than week numbers is mundane but genuinely useful — "week 20" is hard to plan around, whereas "the week of 20 July" tells you whether it clashes with something. It's also what makes it possible to work out when to tell an employer, when maternity leave can start, and whether a planned trip falls inside the window most airlines will still carry you.
The Period Calculator comes first in this chain and explains where the fertile window comes from. The Age Calculator takes over at birth — every appointment for the next two years is specified in weeks, while every relative asks in months, and it answers both. The whole sequence is walked through with consistent dates in the guide on cycle tracking through to your baby's first birthday.
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The standard method (Naegele's rule) adds 280 days, or 40 weeks, to the first day of your last menstrual period. If you know your conception date, the calculator adds 266 days instead.
It is an estimate — only about 1 in 20 babies arrive exactly on the due date. Most are born within two weeks either side. An ultrasound dating scan gives the most accurate estimate.
Pregnancy is divided into three trimesters: first (weeks 1-12), second (weeks 13-27), and third (weeks 28-40). The calculator shows which trimester you are in and key milestones.
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