Naegele's Rule
This calculator uses Naegele's Rule, the most traditional method for estimating the due date: add 280 days (40 weeks) to the last menstrual period (LMP) date. This rule assumes a regular 28-day menstrual cycle with ovulation on day 14 — for irregular cycles or ones different from 28 days, an obstetric ultrasound estimate tends to be more accurate.
The trimesters of pregnancy
Pregnancy is divided into 3 trimesters: the 1st trimester runs through week 13, the 2nd trimester from week 14 to 27, and the 3rd trimester from week 28 until birth (around week 40).
This calculator is only a date-based estimate — prenatal care with an OB-GYN, including ultrasounds, is essential to confirm gestational age and pregnancy health.
Frequently asked questions
Is the calculated due date the exact date the baby will be born?
No — it's an estimate. Fewer than 5% of births happen exactly on the calculated date; most babies are born within a 2-week window before or after the due date, which is considered normal.
Why is gestational age counted from the last period instead of conception?
Because the exact date of conception is rarely known with certainty, while the last menstrual period date is usually remembered more precisely — by medical convention, pregnancy is counted from the LMP, even though conception only happens about two weeks later (at ovulation).