The standard pregnancy due date calculation starts from the first day of your last menstrual period, not from conception. This confuses nearly everyone because it means you are "two weeks pregnant" before the egg is even fertilized. The reason is historical, practical, and still medically sound.
Naegele's Rule
The standard formula, developed by Franz Naegele in the early 1800s, is:
Due date = Last Menstrual Period (LMP) + 1 year - 3 months + 7 days
If your LMP was March 1, 2025:
March 1, 2025 + 1 year = March 1, 2026
March 1, 2026 - 3 months = December 1, 2025
December 1, 2025 + 7 days = December 8, 2025
This assumes a 28-day menstrual cycle with ovulation on day 14. From the LMP, that puts conception at approximately week 2, meaning the first two "weeks of pregnancy" predate conception.
The reason LMP is used instead of conception date is practical: most people can identify the start of their last period with reasonable certainty. The exact date of conception is usually unknown because sperm can survive up to 5 days in the reproductive tract.
Cycle length adjustment
Naegele's Rule assumes a 28-day cycle. If your cycle is consistently longer or shorter, the due date adjusts:
For a 35-day cycle (ovulation around day 21 instead of day 14): Add 7 days to the Naegele's date.
For a 24-day cycle (ovulation around day 10): Subtract 4 days.
The adjustment is: Due date = Naegele's date + (cycle length - 28) days.
This adjustment can shift the due date by a week or more, which matters for scheduling inductions and assessing whether a pregnancy is "overdue."
Ultrasound dating
First-trimester ultrasound dating (crown-rump length measurement between 8-13 weeks) is more accurate than LMP dating. The margin of error is approximately +/- 5 days versus +/- 2 weeks for LMP-based dates.
When the ultrasound date differs from the LMP date by more than 5-7 days, most practitioners revise the due date to match the ultrasound. This is important because it affects decisions about induction timing. An induction at "41 weeks" based on an inaccurate LMP could be at 40 weeks or 42 weeks in reality.
What "due date" actually means
The due date is not a prediction of when the baby will be born. It is the midpoint of the normal delivery window. Only 4% of babies are born on their due date. The normal range for full-term delivery is 37-42 weeks.
The distribution is roughly:
- Before 37 weeks (preterm): 10%
- 37-38 weeks: 15%
- 39-40 weeks: 40%
- 40-41 weeks: 25%
- 41-42 weeks: 10%
Most first pregnancies go past the due date. The average first-time delivery is at 40 weeks and 5 days.
Trimester boundaries
Trimesters are not evenly divided:
- First trimester: Weeks 1-12 (LMP weeks, so really 10 weeks from conception)
- Second trimester: Weeks 13-26
- Third trimester: Weeks 27-40
Each trimester has different developmental milestones and different medical screening schedules. Knowing which trimester you are in determines which prenatal tests and screenings are appropriate.
The calculator
For due date estimation with cycle length adjustment, trimester tracking, and week-by-week developmental information, I built a due date calculator that calculates from LMP, conception date, or ultrasound date and shows the full pregnancy timeline.
I'm Michael Lip. I build free developer tools at zovo.one. 500+ tools, all private, all free.
Top comments (0)