Pregnancy Due Date Calculator

Naegele’s Rule is the internationally accepted standard for estimating a pregnancy due date (Estimated Date of Delivery, or EDD) from the first day of the last menstrual period (LMP). Developed by German obstetrician Franz Karl Naegele in the early 1800s, the rule has remained the clinical standard for over two centuries.

Core formula:

EDD = Last Menstrual Period + 280 days (40 weeks)

Equivalent calculation: EDD = LMP − 3 months + 7 days + 1 year

Variable definitions:

• LMP: first day of the last menstrual period (the starting reference point, even though conception occurs ~2 weeks later)
• 280 days: the standard assumed pregnancy length: 40 weeks from LMP = 38 weeks from conception
• Gestational Age: the age of the pregnancy counted from LMP (not from conception)
• Trimester: pregnancy is divided into three 13-week periods

Worked example: LMP: January 15, 2026. Add 280 days = October 22, 2026 (estimated due date). Using the shortcut: Jan 15 − 3 months = Oct 15, + 7 days = Oct 22, 2026.

At any given date: Gestational weeks = (Today’s date − LMP) / 7

Pregnancy milestone timeline:

Early term is not full term, and the distinction is the point

ACOG split the old “term” bucket in 2013 for a reason worth knowing. Babies born at 37 or 38 weeks have measurably higher rates of respiratory problems, feeding difficulty and NICU admission than babies born at 39 or 40 weeks. It is not a large difference per baby, but it is real, and it is why elective delivery is not normally offered before 39 weeks without a medical reason.

A calculator that reports week 37 as “full term, baby could arrive any day” gets this backwards. This one names the actual band.

Cycle length changes the answer

Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles run 35 days, you ovulate around day 21, so conception happened a week later than the rule assumes and your due date is a week later too. Enter your usual cycle length and the calculator shifts the date by the difference. Leave it blank and it assumes 28.

Accuracy notes

• Only about 4% of babies are born on their exact due date, and roughly 80% arrive within two weeks either side
• A first-trimester ultrasound between weeks 8 and 13 dates a pregnancy more accurately than any calculation from a period date
• If the scan disagrees with the period-based date by more than 5 to 7 days, the scan is what gets used
• Dating gets less accurate later on, so a third-trimester scan does not revise a due date

For information and planning. Confirm the date with your provider.