How is this calculated?
This calculator uses Naegele's rule, the most widely used method, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP) to estimate the due date. If your average cycle is longer or shorter than 28 days, entering it lets the calculator adjust the due date accordingly. Gestational age is calculated by dividing the number of days since your LMP by 7.
How are trimesters divided?
- 1st trimester: week 1 β week 13
- 2nd trimester: week 14 β week 27
- 3rd trimester: week 28 β birth (around week 40)
A note on accuracy
This calculator gives a theoretical estimate based on your last period, and the actual gestational age can differ from this by a few days depending on your real ovulation or conception date. An ultrasound exam from your OB-GYN is the most accurate way to confirm gestational age and due date, so please use this tool for reference only and consult your doctor for an official diagnosis.
Frequently Asked Questions
Why is the last period used instead of the ovulation date?
The actual date of ovulation or conception is often hard to pin down precisely, so medically it's standardized to count gestational age from the first day of the last menstrual period, which is much easier to identify. Because of this, actual conception typically happens about two weeks after the LMP, but gestational age is conventionally counted from the LMP itself.
What if my cycle is irregular?
If your menstrual cycle is irregular, this calculator's accuracy may be reduced. In that case, an early ultrasound exam from your OB-GYN is much more accurate for confirming your due date, so we recommend prioritizing a medical diagnosis.