Estimate the due date and current pregnancy week from LMP, conception, or ultrasound.
Naegele's rule (LMP + 280 days). Variation of ±2 weeks is normal. Confirm with first-trimester ultrasound (CRL).
The first question every newly pregnant person hears is "when are you due?" The answer drives a year of decisions — when to announce, when to take parental leave, which trimester window to schedule the anatomy scan, when to start nesting. The estimate is rarely exact (only 4 % of babies arrive on the projected day; about 70 % land within seven days either side), but it anchors the calendar and synchronises medical care. A precise-enough due date also helps the obstetric team identify pregnancies that are growing too slowly or too fast against expected milestones, since fetal-development metrics are gestational-age-specific. The calculator implements the standard estimation methods used worldwide — Naegele's rule from the last menstrual period (LMP), the +14-day adjustment for known conception date, and the LMP-equivalent value from a first-trimester ultrasound — and returns the projected delivery date, the current pregnancy week, the trimester, and the days remaining.
The dominant convention adds 280 days (40 weeks) to the first day of the last menstrual period: due_date = LMP + 280 days. This is Naegele's rule, in use since 1812, and it implicitly assumes a 28-day menstrual cycle with ovulation on day 14 — meaning conception occurs 14 days after the LMP, and pregnancy lasts 38 weeks from conception (266 days). The calculator handles three input methods: (1) LMP: the date entered IS the LMP, due = LMP + 280 days; (2) conception date (known via IVF or ovulation tracking): the calculator subtracts 14 days to derive the LMP-equivalent and proceeds; (3) ultrasound estimate: a first-trimester crown-rump-length scan provides an LMP-equivalent date directly (the OB sets it from the measured CRL via the Hadlock or Robinson formula), so the calculator treats the input as already-LMP-equivalent. Trimester boundaries are conventional: T1 = weeks 1–13, T2 = 14–27, T3 = 28–delivery. The calculator computes the current week and day from today's date relative to the LMP, the trimester, and days remaining to the projected due date.
Two inputs: the reference method (LMP, conception, or ultrasound estimate) and the reference date. The default — LMP, 1 February 2026 — projects an early November due date. The result panel shows the due date as the headline figure with days remaining alongside, the current pregnancy week (in the standard "weeks + days" notation), the current trimester, and the dates of the trimester transitions. The calculator updates live as the date changes, so women tracking their cycle can replay "if my LMP was actually a week earlier" and see how that shifts the timeline. The dates are formatted in the active site language — French calendars in the FR locale, English in EN.
A woman whose last period started on 1 February 2026 enters that as her LMP. Today is 2 May 2026 — 90 days after the LMP. Due date = 1 February + 280 days = 8 November 2026 (Sunday). Current week: 90 days = 12 weeks + 6 days. Trimester: T1 ends at week 13 (day 91), so the user is on the cusp of T2. Days remaining: 280 - 90 = 190 days. Now consider a different user who tracked her ovulation and knows she conceived on 14 February 2026. Subtract 14 days → LMP-equivalent = 31 January 2026 — only one day off the previous case, due 7 November. A third user had a 12-week ultrasound on 25 April 2026 reporting a CRL-derived gestational age of "12 weeks 0 days". The OB's report converts that to an LMP-equivalent of 1 February 2026, identical to the first case. All three methods converge — that's the system working as intended.
First, picking the wrong reference type. LMP is what most women remember; conception date requires ovulation tracking or IVF records. Mixing them up shifts the projected date by two weeks. Second, applying Naegele's rule to non-28-day cycles. A woman with a 35-day cycle ovulates on day 21, not 14, so adding 280 days from her LMP overestimates the due date by a week. The ultrasound CRL method bypasses this entirely — the scan dates the pregnancy from a measurement, independent of cycle length — which is why most clinicians prefer it for women with irregular cycles. Third, treating the projected date as a deadline. The medical definition of "term" spans weeks 37 through 42, and induction protocols vary by country (UK NICE recommends induction at 41 + 0; France's CNGOF at 41 + 6). Babies arriving in week 37 or 41 are equally normal. Fourth, ignoring discrepancies between LMP-derived and ultrasound-derived dates. If the two disagree by more than seven days in T1, ACOG recommends taking the ultrasound; later in pregnancy the LMP date wins because fetal-size variability widens. Fifth, using the calculator after week 24 as a planning tool. Late-pregnancy bleeding, gestational diabetes, or hypertension can shift the delivery date significantly via planned induction or C-section; the projected date becomes increasingly hypothetical as the medical context evolves.
The 280-day convention is a Western standard; some traditions use lunar months. Mittendorf's rule (1990) found the typical pregnancy in white nulliparous women lasted 288 days, suggesting Naegele's rule underestimates by about a week — but the modern obstetric consensus has stuck with 280 because the variance is too wide to revise the central estimate usefully. Customised due dates — adjusting for maternal ethnicity, parity, height, weight — improve fetal-size assessment but rarely change the headline delivery prediction by more than a few days. Twins and higher-order multiples are typically delivered earlier (median 36 weeks for twins, 32 for triplets), so for a multi-pregnancy the 280-day projection is informational, not a target. IVF pregnancies date themselves from embryo transfer + adjustment for embryo age (3-day or 5-day blastocyst), which the calculator does not handle directly — for IVF, use the conception-date input with the embryo-transfer-day-of-conception adjustment your clinic provides. Pregnancy apps (Flo, Clue, Ovia) implement the same arithmetic and add daily content keyed to the gestational week — they typically use the LMP-with-adjusted-cycle-length method for irregular cycles, then override with the OB's ultrasound date once recorded.