How it's calculated
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. The American College of Obstetricians and Gynecologists (ACOG) puts the average length of pregnancy at 280 days, or 40 weeks, counted from that day.ACOG The estimated due date (EDD) is therefore the LMP plus 280 days.
ACOG also explains that pregnancy is assumed to start 2 weeks after the first day of the LMP, so those first 2 weeks are counted even though you are not yet pregnant.ACOG If you enter a conception date instead, the calculator steps back 14 days to an equivalent LMP, which puts the due date 266 days after conception, the same ovulation-based length NIH researchers use.Human Reproduction
The calculator also lists trimester dates and ACOG's term windows. This is an estimate for planning, not medical advice: your clinician confirms your due date.
The formula
- Due date = first day of LMP + 280 days (or conception date + 266 days)
- Gestational age on any date = days since the LMP, shown as weeks + days (for example 36w 3d)
Trimesters as ACOG defines them:
| Trimester | Gestational age |
|---|---|
| First | 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 |
Term windows from ACOG Committee Opinion 579, endorsed by the Society for Maternal-Fetal Medicine:ACOG
| Term | Gestational age |
|---|---|
| Early term | 37w 0d to 38w 6d |
| Full term | 39w 0d to 40w 6d |
| Late term | 41w 0d to 41w 6d |
| Postterm | 42w 0d and beyond |
The 40-week convention is attributed to the German obstetrician Franz Naegele, whose rule adds nine months and seven days to the first day of the last period.Aust N Z J Obstet Gynaecol Because months have different lengths, the calendar shortcut can land a day away from an exact 280-day count, for example around February in a leap year. The calculator counts the days.
Worked example
If your last period started on August 1, 2026, your estimated due date is May 8, 2027. The second trimester begins on November 7, 2026 and the third on February 13, 2027. Full term runs May 1, 2027 – May 14, 2027.
Counting days across a leap year, a last period starting January 1, 2024 gives a due date of October 7, 2024.
Estimated due date: December 5, 2027 40 WEEKS FROM LMP (CONCEPTION − 2 WEEKS)
Assumptions and limitations
- Few babies arrive on the day. ACOG says only about 1 in 20 women give birth on their due date, and a review of Naegele's rule concludes it should be treated as a guideline, not a definite date.
- Ultrasound can change it. When the LMP date is uncertain, a first-trimester ultrasound may be used to estimate the due date. ACOG notes that once a due date has been selected, it does not change no matter how many more ultrasounds you have. ACOG's guidance for clinicians on dating pregnancies is in Committee Opinion 700.ACOG
- Cycle length. The 280-day count assumes pregnancy begins about 2 weeks after your period started. Variations in the menstrual cycle and the timing of ovulation are among the factors that change the length of a pregnancy. The calculator doesn't adjust for cycle length; ask your clinician if your cycles are long or irregular.
- IVF. Pregnancies from assisted reproduction are dated differently and are not covered by this calculator.
During pregnancy, total water needs rise; the water intake calculator shows the National Academies' values.