Free Pregnancy Due Date Calculator – Estimate Your EDD

How the Pregnancy Due Date Calculator Uses Naegele's Rule

Your estimated due date, written EDD, is the single date around which your baby is most likely to arrive. It is the anchor for everything that follows in antenatal care: when the dating scan is booked, when the anomaly scan falls, when glucose testing happens, when maternity leave starts, and at what point a pregnancy is considered term. This calculator produces that date from the first day of your last menstrual period, then adds your current gestational week and the boundary dates of all three trimesters. Expectant parents use it in the first days after a positive test, before a scan is available. Employers and insurers ask for it. Midwives use it as the reference point that every later measurement is checked against. The method is Naegele's rule, in use since the early nineteenth century and still the standard starting point. In its arithmetic form, EDD = first day of your last menstrual period + 280 days, which is 40 weeks. The classic calendar shortcut is the same idea expressed differently: take the LMP, subtract three months, add seven days and one year. The two can differ by a day or two because calendar months are unequal in length, so this tool uses the 280-day count, which is what most clinical calculators do. The rule assumes a regular 28-day cycle with ovulation near day 14, and it dates pregnancy from your period rather than from conception. That is why you are described as about two weeks pregnant at the moment of conception, and why 40 weeks from LMP corresponds to roughly 38 weeks of actual fetal development. If your cycle runs longer or shorter than 28 days, the tool shifts the date by the difference, so a 32-day cycle adds four days. Take a last period that began on 5 March 2026. Adding 280 days lands the estimated due date on 10 December 2026. The calendar shortcut, March minus three months gives December, plus seven days, would say 12 December, and that two-day gap is exactly the month-length quirk described above. Trimesters follow from the same start point: the first trimester runs through 10 June 2026, which is the end of week 13; the second begins 11 June at week 14 and runs to week 27; the third starts 17 September 2026 at week 28. Full term spans 19 November to 24 December 2026, weeks 37 through 42. A different case: a last period on 10 January 2026 gives an EDD of 17 October 2026, and here both methods agree exactly. Where the date genuinely matters. A woman with a positive test and no scan appointment for three weeks wants to know roughly how far along she is before the first midwife call. A couple planning international travel checks whether week 28, the point most airlines start restricting flights, falls before or after their trip. A teacher works backwards from 10 December to file maternity leave paperwork the required number of weeks in advance. A woman with a 34-day cycle compares the plain 280-day answer against the cycle-adjusted one and sees a six-day difference, then raises it at her dating scan so the sonographer knows her cycle is long. Be clear about how loose this estimate really is. Only about one baby in twenty arrives on the exact due date. Most births happen within two weeks either side, and anything from 37 to 42 weeks is considered term. The biggest source of error is the cycle-length assumption: the LMP method quietly assumes you ovulated on day 14, and for irregular cycles, PCOS, recent contraceptive use, or breastfeeding, that assumption can be off by a week or more in either direction. A first-trimester ultrasound measuring crown-rump length is accurate to within about five days and will override the LMP date if the two disagree by more than a few days, so treat this figure as provisional until a scan confirms it. If you are unsure of your LMP date, say so at your first appointment rather than guessing. These results are general information, not medical advice, and no online calculator replaces antenatal care from your doctor or midwife. The calculation runs entirely in your browser, so the dates you enter are never uploaded or stored.

Pregnancy Due Date Calculator Formula & Method

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