How the due date is estimated
A full-term pregnancy is counted as about 40 weeks from the first day of your last menstrual period (LMP). This calculator uses Naegele's rule: it adds 280 days to your last period date to estimate your due date. If you instead know your conception or ovulation date, it adds 266 days, since conception usually happens about two weeks after the start of your period.
Pregnancy is divided into three trimesters. The first runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. The timeline above marks where you are today. Remember that only about 1 in 20 babies arrive exactly on the estimated date — most are born in the two weeks around it.
By last period vs by conception
If your cycle is not 28 days, the last-period method adjusts for your average cycle length. The conception-date method is useful if you know your ovulation or IVF transfer date and gives a similar estimate.
Frequently asked questions
How accurate is the due date?
It is an estimate. Most babies are born within two weeks before or after the calculated date. An ultrasound dating scan can refine it.
Should I use last period or conception date?
Use your last period date if you know it — it is the standard method. Use conception date if you know your ovulation or IVF transfer date instead.
What if my cycle is not 28 days?
Enter your average cycle length. The calculator adjusts the last-period estimate so longer or shorter cycles are accounted for.
How are trimesters defined?
First trimester is weeks 1–13, second is 14–27, and third is 28 to birth, counted from the first day of your last period.
Is my information private?
Yes. Everything is calculated in your browser. No date you enter is uploaded or stored.