How to calculate your due date
- Choose how to calculate: from the first day of your last period, a known conception date, an IVF transfer date, or an earlier ultrasound.
- Enter the date. For the period method, adjust the cycle length if your cycles are not 28 days.
- See your estimated due date, how many weeks pregnant you are today, the trimester, and a timeline of scans and milestones. Share the link with your partner or family.
How due dates are worked out
Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception, and lasts on average 280 days (40 weeks). This is Naegele's rule, used by doctors and midwives worldwide. If your cycle is longer or shorter than 28 days, ovulation shifts by the same amount, so the due date moves too: a 32-day cycle adds four days. From a known conception date the due date is 266 days later; after IVF it is 261 days after a day-5 (blastocyst) transfer or 263 days after a day-3 transfer.
An ultrasound in the first trimester, which measures the baby's crown-rump length, is the most accurate way to date a pregnancy, and your care team may adjust your due date after it. If you enter your scan result, the calculator dates the pregnancy from that instead.
The due date is an estimate: only about 1 in 25 babies is born on it, and anything from 37 to 42 weeks is normal. Your dates are calculated in your browser and only stored in the link if you share it.
How accurate is a due date calculator?
For regular cycles, LMP dating is usually within about a week or two. An early ultrasound is more precise.
How many weeks pregnant am I?
The weeks are counted from the first day of your last period, so at conception you are already about two weeks pregnant.
Can my due date change?
Yes. It is often updated after the dating scan at 11–14 weeks if the measurements differ by more than about a week.
Is this medical advice?
No. Contact a doctor or midwife to confirm your pregnancy and plan your care.