How due dates are estimated
The most widely used method, known as Naegele's rule, estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period — assuming a 28-day cycle and ovulation around day 14. If your cycle is longer or shorter than 28 days, the calculation can be adjusted accordingly.
This is only an estimate. Babies arrive on a range of dates, and a healthcare provider can give a more precise estimate, particularly using early ultrasound measurements.
The three trimesters
- First trimester — weeks 1 to 12
- Second trimester — weeks 13 to 26
- Third trimester — weeks 27 to 40
Frequently asked questions
How is a pregnancy due date calculated?
The most common method estimates the due date as 280 days (40 weeks) from the first day of the last menstrual period, assuming a typical 28-day cycle.
How accurate is this estimate?
This is a general estimate based on average cycle length. Actual due dates vary, and only about 5% of babies are born exactly on their estimated due date. A healthcare provider can refine this estimate using ultrasound measurements.
What are the three trimesters?
Pregnancy is typically divided into three trimesters: the first trimester covers weeks 1 through 12, the second covers weeks 13 through 26, and the third covers weeks 27 through 40.
Should I rely on this calculator instead of a doctor?
No. This calculator provides a general estimate for informational purposes only. Always consult a healthcare provider for accurate pregnancy dating, monitoring, and care.