Pregnancy Due Date Calculator — Due Date from Last Period

What the 280-day formula assumes — and when it drifts

Naegele's rule adds 280 days to the LMP because it presumes ovulation on cycle day 14 and a 14-day luteal phase, placing conception at day 14 and birth at day 280. For a textbook 28-day cycle that arithmetic is close enough that clinicians worldwide have used it for two centuries. But roughly 20% of people have cycles outside the 26–32 day range, and each day the actual ovulation differs from day 14 shifts the true due date by the same amount. Someone who reliably ovulates on day 21 has a conception window seven days later than the formula assumes, so the real due date is about a week past the Naegele estimate.

The correction is straightforward once cycle length is known: for every day your average cycle exceeds 28 days, add a day; for every day it falls short, subtract one. A 35-day cycle shifts the estimate roughly seven days later; a 21-day cycle shifts it seven days earlier. Your provider can apply this adjustment, and a first- trimester ultrasound at 8–13 weeks will either confirm the date or replace it with a measurement-based estimate. Either way, the LMP calculation is the right starting point — it is the estimate you will carry into your first appointment.

Reading gestational age and trimester from the calculator

The calculator displays gestational age in the same format clinicians use: weeks plus days. "12w 6d" means 12 completed weeks and 6 additional days since the LMP. That is distinct from saying "13 weeks pregnant" — the latter is ambiguous about whether a full week has been completed. Weeks and days together are unambiguous and match what will appear in your prenatal records.

The trimester assignment follows the standard clinical boundaries: first trimester through 13 weeks 6 days (gestational days 0–97), second trimester from 14 weeks through 27 weeks 6 days (days 98–195), and third trimester from 28 weeks onward (day 196+). On April 1, 2026 — 90 days after a January 1, 2026 LMP — the gestational age is 12 weeks 6 days, which lands in the first trimester with 190 days remaining. One week later, at 13 weeks 6 days, the pregnancy is still in the first trimester; the second trimester does not begin until day 98 (exactly 14 weeks of gestation).

Questions

What is the due date for a last period starting January 1, 2026?

October 8, 2026. Naegele's rule adds 280 days to January 1, 2026: January has 31 days, leaving 30 days remaining in January; plus February (28 days in 2026), March (31), April (30), May (31), June (30), July (31), August (31), September (30), and 7 days into October totals 280 days, landing on October 8, 2026. As of April 1, 2026 that pregnancy would be 12 weeks 6 days gestational age, first trimester, with 190 days remaining.

Does the first day of the period mean the first day of bleeding?

Yes. The LMP date used in Naegele's rule is the first day of actual menstrual flow — not the day you felt cramping or spotting the day before, and not a midpoint of the period. Using the wrong day shifts the entire due date estimate by the same number of days. If you are uncertain which day to use, your provider can clarify based on your cycle history or order an early ultrasound for confirmation.

Can I use this if my periods are irregular?

You can use it as a rough starting point, but irregular cycles make the LMP method less reliable. If your cycle length varies by more than a week from month to month, you cannot confidently assume ovulation happened on day 14. In that situation, a first-trimester dating ultrasound is the most accurate way to establish gestational age and an estimated due date. Tell your provider about cycle irregularity at your first prenatal visit.

More ways to use this calculator

Start with the main pregnancy due date calculator or compare the other published scenarios.