Pregnancy Due Date Calculator

What the estimated due date is

An estimated due date, or EDD, is a planning date. The American College of Obstetricians and Gynecologists (ACOG) says that by convention the EDD is 280 days after the first day of the last menstrual period (LMP). It adds that this practice assumes a regular cycle of 28 days, with ovulation on the 14th day after the cycle begins.

The same source says this approach does not account for inaccurate recall of the LMP, irregular cycles or variation in the timing of ovulation. It also reports that approximately one half of women accurately recall their LMP, citing earlier studies.

Two hundred eighty days is exactly 40 weeks. ACOG's patient information says a normal pregnancy lasts about 40 weeks from the first day of the LMP. It notes that pregnancy is assumed to start 2 weeks after that day, so an extra 2 weeks is counted at the beginning, when you are not actually pregnant yet.

Few babies arrive exactly on that date. ACOG says only about 1 in 20 women give birth on their due dates.

ACOG calls ultrasound measurement in the first trimester the most accurate method to establish or confirm gestational age. Your care team decides which date is used for your care.

Weeks, days and trimesters

Gestational age is counted in weeks and days. The patient information from the American College of Obstetricians and Gynecologists (ACOG) gives an example: "36 and 3/7 weeks" means 36 weeks and 3 days. MedlinePlus defines gestational age as measured in weeks from the first day of the woman's last menstrual cycle to the current date.

The calculator divides the number of days since the LMP by 7. The whole number is the weeks, and the remainder is the days. For example, 105 days is 15 weeks and 0 days.

ACOG's trimesters are: first, from the first day of the LMP to 13 weeks 6 days; second, 14 weeks 0 days to 27 weeks 6 days; third, 28 weeks 0 days to 40 weeks 6 days. The calculator labels every day from week 28 onward as third trimester, even after week 40, because it has no separate label beyond that.

Because the count starts at the LMP, the first two weeks are counted before pregnancy has actually begun, as ACOG explains.

Worked examples

These examples use an LMP of March 1, 2026. Term labels come from the American College of Obstetricians and Gynecologists (ACOG). The estimated due date is 280 days later, December 6, 2026.

Computed with the same steps as the calculator on this page. Term labels follow Committee Opinion 579 of the American College of Obstetricians and Gynecologists (ACOG).

Each row adds up: 105 + 175 = 280, and 284 is 4 days after 280. At 40 weeks 4 days the count is still inside ACOG's full term window, which runs through 40 weeks 6 days.

The calculator also shows a second date by Naegele's rule, described in the next section. For this LMP it gives December 8, 2026, two days later than the 280-day date.

Two due dates: 280 days and Naegele's rule

Naegele's rule adds 7 days to the first day of the LMP and then adds 9 months. StatPearls, a reference work hosted by the U.S. National Library of Medicine, describes this as equivalent to 280 days and says it also assumes a regular 28-day cycle with ovulation on day 14.

The two methods can differ by a few days because months have different lengths. In the example above, March 1 plus 7 days is March 8. Nine months later is December 8, while 280 days after March 1 is December 6.

The code handles month ends. If the target month is too short, it uses the last day of that month. For an LMP of May 24, 2026, adding 7 days gives May 31. Nine months later has no February 31, so the calculator uses February 28, 2027. The 280-day date happens to be February 28, 2027 as well.

Both dates are estimates. Your care team's date takes priority, and we show the Naegele date only as a comparison. We could not confirm any source in our set that says one of the two is more accurate than the other.

How sources describe "term" differently

Committee Opinion 579 of the American College of Obstetricians and Gynecologists (ACOG) defines early term as 37 0/7 to 38 6/7 weeks, full term as 39 0/7 to 40 6/7 weeks, late term as 41 0/7 to 41 6/7 weeks, and postterm as 42 0/7 weeks and beyond. The opinion notes that in the past, the period from 3 weeks before until 2 weeks after the due date was considered term.

MedlinePlus uses different wording. It says a normal pregnancy can range from 38 to 42 weeks, that infants born before 37 weeks are considered premature, and that infants born after 42 weeks are considered postmature.

These are different definitions from different organizations, written for different purposes. That is why the calculator shows the ACOG labels and its FAQ points out the MedlinePlus range. A result of "early term" on this page is simply the label in ACOG's table.

Ultrasound can change the date. For a scan before 9 0/7 weeks, ACOG advises changing the EDD when the scan differs from the LMP date by more than 5 days, and before 14 0/7 weeks when it differs by more than 7 days. It sets larger thresholds for later scans. These are guidance, not fixed rules, and clinical judgment also counts.

Edge cases and what to discuss with a professional

The calculator expects the first day of the LMP. If the as-of date is blank it uses today. If the LMP is after the as-of date, it asks you to check the dates. If the LMP is more than 300 days before the as-of date, it also asks you to check.

Irregular cycles are the main limit. The 280-day count assumes a 28-day cycle with ovulation on day 14. The American College of Obstetricians and Gynecologists (ACOG) notes that this practice does not account for irregular cycles or variation in ovulation timing, so a person with irregular cycles can ask a health care professional about dating by ultrasound.

ACOG says that for a pregnancy that resulted from assisted reproductive technology, the gestational age derived from the technology itself should be used. For in vitro fertilization, that means the age of the embryo and the date of transfer. This calculator does not cover these pregnancies.

Questions to ask your care team: which due date they are using and how it was set; whether an early ultrasound is planned; and what the dates mean for the visits you will have. This page is general information, not medical advice.