How pregnancy weeks are counted
Doctors count pregnancy from the first day of your last menstrual period, not from conception. Because ovulation usually happens about two weeks after the period starts, you are already considered "2 weeks pregnant" at conception. A full-term pregnancy is about 40 weeks, or 280 days, from the LMP.
How the due date is calculated
The calculator uses Naegele's rule: due date = first day of the last period + 280 days. A quick way to do it by hand is to add one year, subtract three months and add seven days. Example: if your last period started on January 1, 2026, the estimated due date is October 8, 2026.
Trimesters by week
First trimester: week 1 to the end of week 13. Second trimester: week 14 to the end of week 27. Third trimester: week 28 until birth. A baby born between 39 weeks 0 days and 40 weeks 6 days is considered full term; 37–38 weeks is early term and 41 weeks is late term.
When the estimate can be off
The LMP method assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular, or you do not remember the date, the estimate can be off by a week or more. An early ultrasound, ideally in the first trimester, is the most accurate way to date a pregnancy, and your provider may change your due date based on it. For IVF pregnancies the transfer date gives an exact start.
Only a small share of babies arrive on the due date
Only around 4–5% of babies are born on their estimated due date. Most are born in the two weeks before or after it. Use the date for planning, and follow your doctor or midwife for all medical decisions.