Due Date Calculator
Calculates your estimated due date from your last menstrual period or conception date, adjusting for cycle length and luteal phase. Ideal for early pregnancy planning and prenatal appointment scheduling.
About this calculator
The standard obstetric method (Naegele's Rule) estimates a due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP), assuming a 28-day cycle. This calculator extends that rule: if using LMP, the formula is daysToAdd = 280 + (cycleLength − 28), correcting for cycles longer or shorter than 28 days. If using a known conception date, the formula is daysToAdd = 266 + (lutealPhase − 14), because conception typically occurs about 14 days into the cycle (end of the luteal phase). In both cases, the adjusted day count is added to the reference date to produce the estimated due date (EDD). Only about 5% of babies are born on their exact EDD; the result represents the midpoint of a normal delivery window spanning roughly 37–42 weeks of gestation.
How to use
Suppose your last period began on March 1, 2025, your average cycle is 32 days, and you are using the LMP method. Days to add = 280 + (32 − 28) = 284 days. Add 284 days to March 1, 2025: March 1 + 284 days = December 10, 2025 — your estimated due date. If you instead know your conception date was March 18, 2025 with a 14-day luteal phase: daysToAdd = 266 + (14 − 14) = 266 days. December 9, 2025 — nearly identical, as expected. Enter your own dates to get your personalized EDD instantly.
Frequently asked questions
How accurate is a due date calculated from the last menstrual period?
LMP-based due dates assume a textbook 28-day cycle with ovulation on day 14, which does not apply to everyone. Studies show that only about 4–5% of women deliver on their exact EDD. The estimate is most reliable when cycle length is regular and accurately known. An early ultrasound — ideally before 13 weeks — is considered more accurate for dating a pregnancy because it measures actual fetal size rather than relying on menstrual history.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period and is the standard used by healthcare providers; at conception, gestational age is already approximately 2 weeks. Fetal age (also called embryonic age) is counted from the actual date of fertilization and is therefore about 2 weeks less than gestational age. When your doctor says you are '8 weeks pregnant,' they mean 8 weeks gestational age — your embryo is roughly 6 weeks old. This calculator reports gestational age, which aligns with clinical practice.
Why does cycle length affect the due date calculation?
Ovulation — and therefore the earliest possible conception — typically occurs roughly 14 days before the next expected period, not 14 days after the LMP. If your cycle is 35 days rather than 28, ovulation likely happens around day 21, meaning conception occurred later than Naegele's Rule assumes. Adding (cycleLength − 28) days corrects for this offset, pushing the due date later for longer cycles and earlier for shorter ones. Ignoring cycle length can lead to due dates that are off by a week or more, which has clinical implications for decisions about induction or post-term monitoring.