How to Calculate Your Pregnancy Due Date: Naegele's Rule Explained
Find your exact Estimated Due Date instantly using our Pregnancy Due Date Calculator, or read on to understand how obstetricians calculate your timeline.
Discovering you are pregnant is a life-changing moment. One of the very first questions expectant parents ask is, "When is the baby due?"
While calculating a due date seems like it should be simple, it is based on a standard medical formula that assumes a "perfect" 28-day menstrual cycle. Understanding how this math works helps alleviate anxiety when the timeline inevitably shifts.
The Standard Method: Naegele's Rule
The vast majority of due dates are calculated using Naegele's rule, named after the 19th-century German obstetrician Franz Karl Naegele.
The rule assumes that human gestation lasts approximately 280 days (40 weeks) from the first day of the mother's Last Menstrual Period (LMP).
The Formula:
- Determine the first day of your Last Menstrual Period (LMP).
- Add exactly 7 days to that date.
- Subtract 3 months.
- Add 1 year.
Example Calculation:
- Your LMP was September 10, 2026.
- Add 7 days: September 17.
- Subtract 3 months: June 17.
- Add 1 year: Your Estimated Due Date (EDD) is June 17, 2027.
Why calculate from the LMP?
It seems counterintuitive to start the pregnancy clock before conception actually occurs. However, ovulation and conception are very difficult to pinpoint exactly. The LMP is a physical event that most women can easily date, providing a reliable, standardized starting point for doctors.
Adjusting for Irregular Cycles
Naegele's rule assumes a standard 28-day cycle where ovulation happens exactly on day 14. If your cycle is significantly longer or shorter, your due date calculation must be adjusted.
- If your cycle is 35 days long: You likely ovulate around day 21 (7 days later than the standard model). Therefore, you should add 7 extra days to the standard due date calculation.
- If your cycle is 21 days long: You likely ovulate around day 7 (7 days earlier than the standard model). You should subtract 7 days from the standard calculation.
The Ultrasound Dating Method
While Naegele's rule is the starting point, an early ultrasound is actually the most accurate way to date a pregnancy.
During the first trimester, a sonographer will measure the fetus from head to bottom—known as the Crown-Rump Length (CRL). Because early fetal growth is highly consistent across all human pregnancies, this measurement provides a highly accurate gestational age.
If your ultrasound date differs from your LMP date by more than 5-7 days, your doctor will officially change your due date to match the ultrasound.
FAQ
Are due dates exact? No! An Estimated Due Date is exactly that—an estimate. Only about 4% to 5% of babies are born precisely on their due date. A pregnancy is considered "full term" anywhere between 37 weeks and 42 weeks.
How is IVF pregnancy dated? If you conceived via In Vitro Fertilization (IVF), your due date is the most accurate of all. It is calculated based on the exact date of embryo transfer and the age of the embryo (e.g., a 3-day or 5-day transfer), completely ignoring the LMP.
For a fast, private calculation that accounts for both your LMP and varying cycle lengths, use the CalcUnit Pregnancy Due Date Calculator.