How your due date is calculated
Pregnancy is counted as 40 weeks (280 days) from the first day of your last menstrual period, a method known as Naegele's rule. It assumes a 28-day cycle with ovulation around day 14, so this calculator shifts the date if your cycle is usually longer or shorter.
If you know your conception date, your due date is 266 days (38 weeks) later. For IVF, it's counted from the transfer date: 261 days after a day-5 blastocyst transfer, or 263 days after a day-3 transfer.
How accurate is a due date?
A due date is an estimate, not a deadline. Only a small share of babies, roughly 1 in 20, arrive on the exact day, and most are born somewhere between 37 and 42 weeks. A first-trimester ultrasound is the most accurate way to date a pregnancy, so don't be surprised if your provider updates your date after your first scan.
Due date questions
Is my due date counted from conception?
No. It's counted from the first day of your last period, about two weeks before conception usually happens. That's why you're considered about 4 weeks pregnant around the time of a missed period.
What if my cycles are irregular?
Period-based dates are less reliable with irregular cycles. An early ultrasound gives a better estimate, and your provider will usually go by that.
Can my due date change?
Yes. Providers often adjust it after a first-trimester ultrasound if the baby's measurements differ from your period-based date by more than about a week.
When should I pack my hospital bag?
Most providers recommend having your bag ready by 36 weeks. Here's my real hospital bag guide.
When should I hire a doula?
Many families start looking in the second trimester, which leaves time to meet, plan, and do prenatal visits together. It's never too late to reach out, even in your final weeks.