Due date calculator: when is your baby due?

How your due date is calculated

Most due date calculators, including this one, use Naegele's rule: your due date is 280 days (40 weeks) after the first day of your last menstrual period. Pregnancy is counted from that day, about two weeks before you actually conceive, so at the moment of conception you're already counted as "2 weeks pregnant".

The rule assumes a 28-day cycle. If your cycles are longer or shorter, ovulation usually happens later or earlier, so the calculator moves your due date by the difference. With a known conception date it adds 266 days. After IVF it counts from the transfer date: 261 days for a 5-day embryo (blastocyst) and 263 days for a 3-day embryo.

How accurate is a due date?

A due date is an estimate, not an appointment. Only about 1 in 20 babies is born on the day itself. Most arrive between 37 and 42 weeks. In medical terms, 37+0 to 38+6 weeks is early term, 39+0 to 40+6 is full term, 41 weeks is late term and 42 weeks or more is post-term.

An ultrasound in the first trimester, which measures your baby from crown to rump, is the most accurate way to date a pregnancy. If it differs enough from your period-based date, your care team will use the scan date instead.

What the weeks mean

Weeks are usually written as completed weeks plus days. "24+3" means 24 weeks and 3 days. The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until your baby is born. See what's happening each week in the pregnancy week by week guide.