Due Date Calculator Based on Last Period: When Will Your Baby Arrive?

How is the estimated due date calculated?

The most common method is Naegele's rule: 280 days (40 weeks) are added to the first day of your last period. Adding 7 days and counting back 3 months gives the same result. For example, for a woman whose last period started on January 10th, the estimated due date is around October 17th (except in leap years).

This method assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer than 28 days, our tool reflects the difference in the date: a 32-day cycle shifts the estimated due date 4 days later, a 25-day cycle shifts it 3 days earlier.

The first day of bleeding is used as the last period date; spotting doesn't count as the start. In the periods right after childbirth, while breastfeeding, or after stopping a hormonal method, cycles can be irregular, so the calculated date may deviate from the actual one in these cases.

What should you watch for as your due date approaches?

As the estimated date approaches, it helps to pack your hospital bag, plan your transportation, and know the signs your doctor has described. If you experience regular, increasingly frequent contractions, your water breaking, vaginal bleeding, or a noticeable decrease in baby movements, contact your doctor or the nearest health facility without waiting for the due date. If your pregnancy is high-risk, your doctor may plan the timing and method of delivery separately.

Is the baby born exactly on the estimated date?

No — the estimated due date is the midpoint of a range, not a single day. Only a small share of babies are born exactly on that day. Births before week 37 are called preterm, births between weeks 37 and 42 are considered full-term, and births after week 42 are considered post-term. This is why monitoring by your doctor and tracking symptoms matters as the date approaches.

When does the date become certain?

A calculation based on the last period date is an estimate. The due date is confirmed by ultrasound measurement, especially in the first trimester; if there's a significant difference between the measurement and the calculation, your doctor updates the date based on the ultrasound. This measurement is especially important for women who don't remember their last period date or who have irregular cycles.

This tool is not a medical diagnosis; consult an obstetrician for your birth plan and pregnancy tracking.