How the due date is calculated
The standard method is Naegele's rule — add 280 days (40 weeks) to the first day of your last menstrual period:
Due date = First day of last period + 280 days (± cycle adjustment)
If your cycle isn't 28 days, ovulation shifts, so the calculator adds the difference between your cycle length and 28. Conception is estimated around ovulation — roughly your cycle length minus 14 days after the period began.
How far along you are
Gestational age is counted in weeks and days from the first day of your last period to today. The trimesters break down as:
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: week 28 to birth
It's an estimate
Only about 5% of babies arrive on the exact due date, and most come within two weeks either side. An early ultrasound gives the most accurate dating, especially with an irregular or unknown cycle.
Frequently asked questions
How is a due date calculated?
Most due dates use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Equivalently, subtract three months from the LMP and add seven days. This calculator also adjusts for cycle lengths other than 28 days.
Why is pregnancy dated from the last period, not conception?
Because the LMP is a date most people can pinpoint, while the exact moment of conception usually is not. Dating from the LMP makes 40 weeks the standard length, even though fertilization typically happens about two weeks later, around ovulation.
How accurate is a due date?
It is an estimate — only about 1 in 20 babies arrive on the exact due date. Most births happen within two weeks either side. An early ultrasound can refine the date, especially if your cycle is irregular or its length is unknown.
What if my cycle is not 28 days?
Ovulation shifts with cycle length, so the due date shifts too. This calculator adds the difference between your cycle length and 28 days. A 32-day cycle, for instance, pushes the estimated due date about four days later.
How are the trimesters divided?
By convention, the first trimester runs through week 13, the second from weeks 14 to 27, and the third from week 28 until birth. The calculator shows which trimester today falls in based on your current gestational age.
Disclaimer: This calculator provides an estimate based on Naegele's rule and is not a substitute for medical care. Confirm dating and any concerns with your doctor or midwife.