Due Date Calculator — Pregnancy Due Date, Conception & Gestational Age

You took the test, you saw the second line, and now you want a date to circle on the calendar. For a naturally conceived pregnancy — where you don't know the exact moment of conception — clinicians in the United States start the count from a date you almost certainly remember: the first day of your last menstrual period (LMP). The method is called Naegele's rule, named for the 19th-century German obstetrician who popularized it, and it's the foundation of every paper pregnancy wheel and prenatal chart you'll encounter. This page owns that LMP-based math; if you conceived through fertility treatment, the dating works differently and you'll want the IVF Due Date Calculator instead, which counts from your embryo transfer day rather than a period you may not have tracked.

Naegele's rule sets a full pregnancy at 280 days — exactly 40 weeks — measured from the first day of the LMP, not from the day you actually conceived. That's why the math feels strange at first: you're labeled "2 weeks pregnant" before ovulation has even happened, because the clock starts roughly two weeks ahead of fertilization. The base calculation is just due date = first day of LMP + 280 days. A period that began on March 1 plus 280 days gives an estimated due date (your EDD) of December 6. The classic bedside shortcut is to take the LMP, walk back 3 months, add 7 days, and roll the year forward — handy for a quick guess, but because it pretends every month is 30 days it can drift a day or two. This tool counts real calendar days, so it stays exact and never stumbles over a leap-year February 29.

The 280-day figure quietly assumes a 28-day cycle with ovulation on day 14, and plenty of bodies don't read that memo. When your cycle is longer or shorter, you ovulate later or earlier, so the due date slides by the same amount: adjusted EDD = LMP + 280 + (cycle length − 28). A 31-day cycle nudges the date 3 days later; a 25-day cycle pulls it 3 days earlier. Enter your real average length and the tool applies that shift automatically.

Worked example. Your LMP was March 1, 2026 and your cycles average 31 days. Base EDD = March 1 + 280 = December 6, 2026; the cycle adjustment of +3 days moves it to December 9, 2026. Estimated conception falls around ovulation — roughly March 17, 2026 once the cycle adjustment is folded in. If today is June 19, 2026, that's 110 days past the LMP, which works out to about 15 weeks and 5 days, squarely in the second trimester.

The error that catches almost everyone is counting from conception instead of the LMP. Since the medical world dates from your last period, your "weeks pregnant" figure already runs about two weeks ahead of the embryo's biological age — do not shave those weeks off to "correct" it, or every chart and appointment will be out of sync.

This is general information, not medical advice. Roughly 1 baby in 20 actually shows up on the predicted day; most arrive within the two weeks bracketing it. At your first scan your provider compares the LMP estimate against the baby's measured size and may reset the EDD — that ultrasound-based date becomes the one your care is built around.