The estimated due date (EDD) is 280 days, or 40 weeks, after the first day of the last menstrual period. That one convention sets every later date in pregnancy care, from the timing of screening tests to the definition of preterm birth. This calculator works from the last period, a known conception date or an ultrasound, adjusts for cycle length, and shows the gestational age today and the term window.
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EDD = first day of the last menstrual period + 280 days. The quick version, Naegele’s rule, is to add 7 days to the LMP, subtract 3 months, and add 1 year. An LMP of 10 January 2026 gives a due date of 17 October 2026.
Gestational age is counted from the LMP, not from conception, so at conception a pregnancy is already about "2 weeks" along. That convention assumes a 28-day cycle with ovulation on day 14. With a longer or shorter cycle, ovulation moves, and a common adjustment shifts the EDD by the difference: a 35-day cycle adds 7 days. The calculator applies this when you enter a cycle length other than 28.
Naegele’s shortcut and adding 280 days can disagree by a day or two, because months have different lengths. The 280-day count is the standard. Recall is the weak point: ACOG notes that only about half of women accurately remember the date of their last period.
A first-trimester ultrasound, up to and including 13 weeks 6 days, is the most accurate way to establish or confirm gestational age. When the scan and the LMP disagree, ACOG recommends changing the due date to the ultrasound date if the difference is larger than the following:
| Gestational age at scan (by ultrasound) | Redate if the difference is more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days or later | 21 days |
Once a due date has been set from the best information available, it should not keep moving with later scans. Late scans are poor at dating because babies grow at different rates. Pregnancies from IVF are dated from the embryo age and transfer date instead of the LMP.
| Category | Gestational age |
|---|---|
| Preterm | before 37 weeks 0 days |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks 0 days or later |
ACOG replaced the old single "term" label (37 to 42 weeks) with these categories in 2013, because outcomes for babies born at 37 and 38 weeks are measurably different from those born at 39 and 40. Gestational age is written as weeks plus days: 38+4 means 38 weeks and 4 days.
| Milestone | Gestational age (approximate) |
|---|---|
| Pregnancy test usually positive | around 4 weeks |
| Cardiac activity seen on transvaginal ultrasound | about 6 weeks |
| End of the first trimester | 13 weeks 6 days |
| Anatomy (anomaly) scan | 18–22 weeks |
| Third trimester begins | 28 weeks |
| Early term | 37 weeks |
| Due date | 40 weeks 0 days |
| Post-term | 42 weeks |
Trimester boundaries are conventions and differ slightly between sources. Scan timing also follows local schedules, so a clinic may offer the anatomy scan at 20 weeks or anywhere in that range.
The EDD marks the middle of a window, not an appointment. Only a small share of babies, often quoted at around 4 to 5 percent, are born on the day itself. Most are born in the weeks either side, and studies of uncomplicated pregnancies have found that the average length of pregnancy can run a few days past 280, especially in first pregnancies. That is one reason ACOG treats 41 weeks as late term, not overdue.
Add 280 days (40 weeks) to the first day of the last menstrual period. The Naegele shortcut is to add 7 days, subtract 3 months and add 1 year. This assumes a 28-day cycle; with a longer cycle, the due date is usually moved later by the extra days.
A first-trimester ultrasound, up to 13 weeks 6 days, is the most accurate method. ACOG recommends changing the due date to the ultrasound date when the two differ by more than set limits: more than 5 days before 9 weeks, more than 7 days from 9 to 15 weeks 6 days.
In the ACOG definitions, full term is 39 weeks 0 days to 40 weeks 6 days. Early term is 37 weeks 0 days to 38 weeks 6 days, late term is 41 weeks, and post-term is 42 weeks or later. Birth before 37 weeks is preterm.
From the first day of the last menstrual period, not from conception. Conception usually happens about two weeks later, so at conception a pregnancy is already counted as roughly 2 weeks. Gestational age is written as weeks plus days, such as 32+5.
Only a small minority, commonly quoted as around 4 to 5 percent. The due date is the middle of a window: most babies arrive within a couple of weeks either side of it.