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Free tool · no sign-upEnter your last period date to find out how many weeks pregnant you are, which trimester you’re in, and your estimated due date — in seconds.
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Pregnancy is counted not from the day of conception but from the first day of your last menstrual period (LMP). That’s because it’s usually impossible to know the exact day conception happened, while the last period gives a clear starting point. This method is called Naegele’s rule and is used as the standard worldwide.
The estimated due date is found by adding an average of 280 days (40 weeks) to your last period. If your cycle is longer or shorter than 28 days, ovulation shifts, so the calculation fine-tunes accordingly.
Pregnancy is divided into three periods (trimesters): the first trimester covers the first 13 weeks, the second trimester weeks 14–27, and the third trimester from week 28 to birth. Each has its own developmental stages and check-ups. The week and date you see are estimates — your body and your baby follow their own rhythm.
What if I don’t know my exact last period date?
You can enter an approximate date; the result will be approximate too. If your cycles are irregular or you are unsure of the date, an early-pregnancy ultrasound gives the most accurate gestational age — ask your doctor.
How accurate is this calculation?
It is based on an average 280-day pregnancy (Naegele’s rule) and is an estimate. Only a small share of babies arrive on the exact due date; most are born a few days before or after.
Does the result change if my cycle is not 28 days?
Yes. If you enter your cycle length, the calculation adjusts. With longer cycles ovulation happens later, so the estimated due date shifts a few days forward.
Which date should I use if I conceived through IVF?
For IVF the date is based on the embryo transfer day, not your last period. Use the due date calculator or the date your doctor gave you.
This calculation is based on Naegele’s rule (from your last period) and on ACOG / FIGO 2018 criteria for the trimester definitions. Results are estimates and don’t replace medical advice; the exact gestational age is confirmed by an early-pregnancy ultrasound. If anything worries you, talk to your doctor.
Other tools: Due date calculator · Ovulation calculator · Period calendar