Calculate your estimated due date from your last menstrual period, conception date, or IVF transfer date — with current pregnancy week, trimester, milestones, and a countdown to your due date.
The single most important thing to understand about any pregnancy due date is that it is a statistical estimate representing the midpoint of a normal range, not a precise prediction of when labor will begin. Full-term delivery is medically defined as occurring anywhere between 37 and 42 completed weeks, meaning a healthy, entirely normal birth can happen up to three weeks before or two weeks after the calculated date. Only a small fraction of pregnancies — roughly 1 in 20 — actually deliver on the exact calculated date.
This calculator gives you the same estimate a care provider would calculate using the same standard formulas (Naegele's rule for LMP, the 266-day rule for conception, and clinic-standard offsets for IVF transfers), but it's worth treating the result as a planning anchor — useful for scheduling prenatal appointments, tracking fetal development milestones, and general preparation — rather than a fixed date to build rigid plans around.
Naegele's rule is named after Franz Karl Naegele, a 19th-century German obstetrician who formalized the '280 days from LMP' calculation, though the underlying idea predates him. The rule works on two combined assumptions: that ovulation and conception occur around day 14 of a standard 28-day menstrual cycle, and that a typical pregnancy lasts 266 days from conception. Adding those together (14 days to conception + 266 days of pregnancy) gives the familiar 280-day, or 40-week, figure counted from the first day of the last period.
The practical implication is that this method is most accurate for people with regular, close-to-28-day cycles, and progressively less precise for people with notably longer or shorter cycles, since ovulation timing shifts accordingly. Some care providers adjust the calculation for known cycle length by adding the actual cycle length minus 28 days to the standard estimate — this calculator uses the standard 280-day convention, which remains the most widely used default across clinical practice.
When the exact conception date is known — through ovulation tracking, a fertility monitor, or a single recorded instance of intercourse — the 266-day-from-conception method removes the cycle-length assumption entirely, generally producing a tighter estimate than LMP-based dating for anyone whose cycles don't fall neatly at 28 days. IVF dating takes this precision a step further: because the fertilization date and the embryo's exact developmental stage at transfer (day 3 cleavage-stage or day 5 blastocyst) are both known with medical certainty, IVF due dates are considered among the most reliable due-date estimates available, on par with or exceeding early-ultrasound dating.
Fertility clinics universally use the transfer-date-plus-offset convention (261 days for a day-5 transfer, 263 for a day-3 transfer) precisely because it reflects the embryo's true developmental age rather than an assumption about a hypothetical last period that, for an IVF pregnancy, isn't really the relevant reference point at all.
Pregnancy is conventionally measured in weeks and days from the LMP-equivalent starting point (day 0), regardless of which method was used to establish the due date — this is why every method in this calculator, once it produces a due date, converts back to an equivalent LMP reference date for week counting, keeping the week and trimester figures consistent no matter which input method you used. 'Week 20, day 3' means 20 complete weeks and 3 additional days have passed since that reference date.
This week-and-day convention is what care providers use throughout prenatal care — appointment scheduling, screening test windows (like the first-trimester screen or 20-week anatomy scan), and developmental milestones are all referenced by gestational week rather than calendar date, since gestational week is what actually correlates with fetal development stage, independent of when in the calendar year the pregnancy happens to fall.
The milestone dates this calculator generates — end of first trimester, anatomy scan window, viability, start of third trimester, full term, and due date — mark points that are clinically meaningful rather than arbitrary. The first-trimester boundary at week 13 roughly marks the point where miscarriage risk drops substantially and many people choose to share pregnancy news. The 20-week anatomy scan window is when detailed fetal ultrasound imaging typically occurs, often when sex can be determined. Week 24 marks the viability threshold referenced earlier. Week 28 begins the third trimester, when appointment frequency typically increases. Week 37 marks 'full term,' after which labor is no longer considered preterm.
Having these dates calculated in advance makes it easier to know what prenatal appointments and screenings to expect and when, and gives a concrete sense of how far along a pregnancy is relative to these well-known clinical benchmarks.
Pregnancy Due Date Calculator estimates your due date and tracks pregnancy progress. These related calculators cover other date and health calculations.