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Estimate your baby’s due date from the first day of your last period — plus your current week, trimester and likely conception date.
The Pregnancy Due Date Calculator above estimates your baby’s arrival from just two inputs. Follow these steps:
Tip: If you are not sure of your exact LMP date, enter your best estimate and treat the result as a range: most babies arrive within two weeks on either side of the estimated date. Confirm the dating with your healthcare provider, ideally with a first-trimester ultrasound.
Pregnancy dating means assigning a gestational age — how far along a pregnancy is — measured in weeks and days from the first day of the last menstrual period. This convention surprises many people: at the moment of conception, you are already considered two weeks pregnant, because dating starts before ovulation. Fetal age, the true age of the embryo, is therefore always about two weeks less than gestational age.
Gestational age is the backbone of prenatal care. It determines when screening tests are scheduled (for example, the nuchal translucency scan around weeks 11–13), when viability milestones fall (around 24 weeks), and whether a birth is classified as preterm (before 37 weeks), early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), or post-term (42 weeks and beyond). Every clinical decision — from induction timing to steroid administration — leans on this number.
Pregnancy is divided into three trimesters: weeks 1–13 (first), weeks 14–27 (second), and weeks 28–40 (third). The first trimester covers embryonic development and the highest miscarriage risk; the second is often the most comfortable, with anatomy scans around week 20; the third brings rapid fetal growth, now about 200–250 grams per week, and final preparations for birth.
Dating by LMP works best for women with regular cycles who know their last period date. When cycles are irregular, the LMP is uncertain, or the date is simply unknown, ultrasound dating takes over: a first-trimester crown-rump length measurement dates a pregnancy to within about 5 days, making it the gold standard. In practice, clinicians use LMP dating first and revise it when an early ultrasound disagrees by more than 5–7 days.
The calculator uses Naegele’s rule, the classic obstetric formula named after 19th-century German obstetrician Franz Naegele:
Estimated due date = LMP + 280 days + (cycle length − 28)
The 280 days equal 40 weeks — the average length of human gestation from the last period. The correction term shifts the date for cycles that differ from the textbook 28 days, because ovulation (and therefore conception) occurs later in longer cycles and earlier in shorter ones. Likely conception is estimated as LMP + 14 days for a 28-day cycle, adjusted similarly for other lengths.
Worked example 1: LMP = January 1, 2026, 28-day cycle. Add 280 days: January has 30 days remaining after the 1st, then February 28, March 31, April 30, May 31, June 30, July 31, August 31, September 30 — that accounts for 272 days — leaving 8 days into October. Estimated due date: October 8, 2026. Likely conception: January 1 + 14 days = January 15, 2026. The cycle correction is (28 − 28) = 0, so no adjustment.
Worked example 2: LMP = March 10, 2026, 32-day cycle. Start with 280 days from March 10: 21 days remain in March, then April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30 — totaling 265 days — leaving 15 days into December: December 15. Now apply the cycle correction: (32 − 28) = +4 days. Estimated due date: December 19, 2026. Likely conception falls around ovulation, roughly day 16 of a 32-day cycle: March 10 + 16 days = March 26, 2026. Notice how a 4-day-longer cycle pushes the due date 4 days later — ovulation timing, not period timing, is what truly matters.
Naegele’s rule assumes ovulation on day 14 and a textbook cycle, which is why the cycle-length correction — and ultrasound confirmation — exist. It is an estimate, not an appointment: only about 5% of babies arrive on their due date.
Take the widget’s default scenario: LMP January 1, 2026, 28-day cycle, estimated due date October 8, 2026. What does this date actually tell you? It marks the point of 40 weeks 0 days gestation — the middle of the full-term window (39 weeks 0 days through 40 weeks 6 days), when neonatal outcomes are statistically best.
But treat it as the center of a range, not a deadline. Roughly 80% of babies arrive between 37 and 42 weeks — that is, between September 17 and November 5, 2026 for this pregnancy. About 10% arrive before 37 weeks (preterm) and a smaller share after 42 weeks (post-term, when providers usually recommend induction). First babies are slightly more likely to arrive a few days “late,” averaging around 40 weeks 5 days in some studies.
Practically, use the due date to count backward: maternity leave paperwork, the 20-week anatomy scan (around May 21, 2026), third-trimester glucose screening around week 28 (July 16, 2026), and weekly checkups from week 36 (September 10, 2026). The calculator’s week-by-week readout keeps you oriented at every stage.
Using the same pregnancy (LMP January 1, 2026), here is how the trimesters map to the calendar. First trimester: weeks 1–13, January 1 through April 1, 2026 — the embryo becomes a fetus at week 9, and the nuchal translucency screening falls around March 19–26. Second trimester: weeks 14–27, April 2 through July 8, 2026 — the anatomy scan lands around May 21, and fetal movement (“quickening”) typically appears between weeks 18 and 22. Third trimester: weeks 28–40, July 9 through October 8, 2026 — glucose screening around July 16, weekly visits from September 10, and full term beginning September 24.
On October 7, 2026 — the day before the due date — the calculation is: 279 days elapsed ÷ 7 = 39 weeks with 6 days remainder, so 39 weeks, 6 days, firmly in the third trimester with about 1 day to go. This weeks-plus-days format is how clinicians always communicate progress, because a single week contains enormous developmental change in late pregnancy.
Each trimester also brings different guidance: folic acid and first-trimester screening early on; anatomy review and gestational diabetes screening mid-pregnancy; kick counts, Group B strep testing (around week 36), and birth planning late. Your provider personalizes this timeline — the calculator keeps you synced to it.
Support a healthy pregnancy with these tools:
This page hosts a free Pregnancy Due Date Calculator. Inputs: first day of last menstrual period (LMP, date) and average cycle length (days, default 28). Outputs: estimated due date, likely conception date, current gestational age (weeks + days), and trimester. Method: Naegele’s rule — EDD = LMP + 280 days + (cycle length − 28); conception ≈ LMP + 14 days (adjusted for cycle length). Example: LMP January 1, 2026, 28-day cycle → EDD October 8, 2026, conception ≈ January 15, 2026. Trimesters: weeks 1–13, 14–27, 28–40. Only ~5% of births occur on the estimated date; most fall within ±2 weeks. The page also explains gestational vs fetal age, trimester milestones, and common dating mistakes.
Enter the first day of your last menstrual period (LMP) and your average cycle length. The calculator adds 280 days (40 weeks) to the LMP using Naegele’s rule, adjusted if your cycle differs from 28 days.
Formula: Due date = LMP + 280 days + (cycle length − 28). Conception ≈ LMP + 14 days.
With Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. An LMP of January 1, 2026 gives a due date of October 8, 2026.
They estimate within about 1–2 weeks for most women; only around 5% of babies arrive exactly on their due date.
Enter your average cycle length — the calculator shifts the due date by the difference from a 28-day cycle.
Roughly 14 days after the LMP in a 28-day cycle.
Weeks 1–13 first, 14–27 second, 28–40 third. The calculator shows your current week and trimester automatically.
Yes. If a first-trimester ultrasound (crown-rump length) disagrees with LMP dating by more than 5–7 days, your provider will usually revise the due date to the ultrasound estimate, which is accurate to about ±5 days. Later ultrasounds are less precise for dating, so the earliest scan carries the most weight.
A measurement 1–2 weeks off the expected size is usually normal variation — babies grow at slightly different rates, and ultrasound estimates have a margin of error. Consistently large or small measurements across visits may prompt your provider to re-check the due date or monitor growth more closely.
No. The rule adjusts automatically: add (your cycle length − 28) days to the standard 280-day count. A 35-day cycle shifts the due date 7 days later than the 28-day estimate, because ovulation — and therefore conception — happens about a week later. Enter your true average cycle length in the calculator above.
Sources: American College of Obstetricians and Gynecologists (ACOG) guidance on pregnancy dating; National Institutes of Health (NIH) pregnancy resources; World Health Organization (WHO) maternal health guidance.
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