How to calculate pregnancy due date correctly.
Knowing your estimated due date (EDD) is one of the first and most important milestones in pregnancy. It shapes your prenatal care schedule, helps you plan for birth, and gives you a timeline for the incredible journey ahead. But how is it actually calculated — and how accurate is it?
Naegele's Rule: the standard method
The most widely used method for calculating due dates is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. The formula is simple:
EDD = First day of last menstrual period (LMP) + 280 days (40 weeks)
Or equivalently: take the first day of your LMP, subtract 3 months, and add 7 days. This assumes a 28-day menstrual cycle with ovulation on day 14.
Why 40 weeks?
The 40-week count starts from the first day of your last period — not from conception. This means the first two weeks of "pregnancy" technically occur before you've even conceived. Actual fetal development begins around week 3.
This convention exists because the LMP is a known date, while the exact moment of conception is usually unknown. It's a practical approximation that has been used in obstetrics for over 200 years.
Adjusting for cycle length
Naegele's Rule assumes a 28-day cycle. If your cycle is longer or shorter, your actual ovulation — and therefore conception — happens at a different time.
For example, if you have a 35-day cycle, you likely ovulate around day 21 (not day 14). This means your EDD should be adjusted forward by 7 days.
Our Due Date Calculator automatically adjusts for your cycle length, giving you a more accurate estimate than the standard formula.
How accurate is any due date calculation?
Only about 4% of babies are born on their exact due date. Most births occur within a 2-week window on either side. The EDD is best understood as the midpoint of a range, not a precise target.
Ultrasound dating (especially in the first trimester) is generally more accurate than LMP-based calculation, because it measures the actual size of the embryo. If your ultrasound date differs from your LMP date by more than a week, your doctor may adjust your EDD.
What the trimesters mean
First trimester (Weeks 1–12): Organ formation, highest risk of miscarriage, morning sickness common.
Second trimester (Weeks 13–26): Often called the "honeymoon phase" — energy returns, belly grows, you may feel the baby move.
Third trimester (Weeks 27–40): Rapid growth, preparation for birth, increased discomfort as baby runs out of room.
Your due date is a guide, not a guarantee. Use it to plan your prenatal care and prepare for birth, but stay flexible — babies arrive on their own schedule. Our Due Date Calculator gives you the most accurate estimate possible based on your cycle, but always confirm with your healthcare provider.