What an estimated due date (EDD) means

The Estimated Due Date (EDD) is the date you're most likely to give birth — typically calculated as 40 weeks (280 days) from the first day of your last menstrual period (LMP). It's an estimate: only about 5% of babies are born on their exact due date. Most arrive between 37 and 42 weeks, with a normal range of about ±2 weeks around the EDD.

The EDD is the anchor for prenatal care timing. Every appointment, ultrasound, screening test, and decision about delivery is scheduled relative to it.

The three ways to calculate EDD

1. Naegele's rule (from LMP)

The traditional formula, developed by Franz Karl Naegele in 1812:

EDD = LMP + 280 days  (i.e. LMP + 1 year − 3 months + 7 days)

Assumes a regular 28-day cycle with ovulation on day 14. About 80% of women with regular cycles will deliver within ±2 weeks of this date.

For cycles longer or shorter than 28 days, adjust: add the number of days your cycle exceeds 28 (or subtract if shorter). The calculator does this automatically when you enter your cycle length.

2. From conception date

If you know when you conceived (e.g. you tracked ovulation):

EDD = conception date + 266 days  (38 weeks)

More accurate than LMP when conception is precisely known. The 266 vs 280 day difference is the ~2-week gap between LMP and ovulation.

3. From IVF embryo transfer

For IVF pregnancies, conception is precisely known:

EDD = transfer date + 263 days  (if Day 5 blastocyst)
EDD = transfer date + 266 days  (if Day 3 embryo)

IVF EDDs are the most accurate because the conception moment is recorded.

What this calculator gives you

• EDD computed by all three methods (you pick the one you have data for).
• Current gestational age in weeks + days ("24 weeks 3 days").
• Current trimester (1st: weeks 1–13, 2nd: 14–27, 3rd: 28–40+).
• Key milestones with dates: first heartbeat (~6 weeks), end of first trimester (13w), anatomy scan window (18–22w), viability (24w), full term (37w).
• Countdown to EDD in days/weeks.
• A shareable URL so you can send the same calculation to your partner / doctor / family.

Important caveats

• Ultrasound dating is more accurate than LMP-based dating, especially in the first trimester. If your OB has done a crown-rump-length scan, the date they assigned supersedes any home calculation.
• Late discoveries. If you don't know your LMP (irregular cycles, breastfeeding pre-pregnancy, recent pill use), an early ultrasound gives the most accurate dating.
• Multiples. Twin and higher-order pregnancies typically deliver earlier (twins ~37 weeks, triplets ~33), but the EDD is still calculated the same way — your provider will plan around the actual numbers.
• This is educational reference. Pair it with regular prenatal care; don't substitute it for your OB's clinical assessment.