Due Date Calculator

Obstetric dating counts in gestational age, measured from the first day of the last menstrual period, even though fertilisation has not occurred at that point. The convention exists because the LMP is an event a patient can usually recall, whereas conception is rarely observed. Naegele's rule adds 280 days, or 40 weeks, to that date, which embeds two assumptions: a 28-day cycle and ovulation on day 14. Where conception is known, as in IVF or a documented ovulation date, the interval is 266 days, and this tool back-calculates an equivalent LMP by subtracting 14 days so that weekly milestones stay on the standard gestational scale. Fetal or conceptional age therefore runs about two weeks behind the gestational age quoted in clinic. The due date is a central estimate, not a deadline. Roughly 4 to 5 percent of births occur on the estimated date itself, while about 80 percent fall between 37 and 42 weeks. Current terminology, standardised by the American College of Obstetricians and Gynecologists, divides this window into early term at 37 0/7 to 38 6/7 weeks, full term at 39 0/7 to 40 6/7, late term at 41 0/7 to 41 6/7, and postterm from 42 0/7. The dominant limitation is cycle variability: irregular cycles, polycystic ovary syndrome, recent hormonal contraception, or a late ovulation all shift the true conception date away from day 14 and carry the LMP-based estimate with them. First-trimester ultrasound measurement of crown-rump length dates a pregnancy to within about 5 to 7 days and is used to override the LMP estimate when the two disagree by more than that margin.

The 280-day rule is named for Franz Karl Naegele, professor of obstetrics at Heidelberg from 1810 until his death in 1851, who set it out in his obstetrics textbook of 1812. Naegele was formalising an older clinical observation that human pregnancy spans roughly ten menstrual cycles rather than deriving anything new from measurement, and his phrasing left a lasting ambiguity about whether the count should begin on the first or the last day of the menstrual period. The convenient arithmetic form still taught, subtract three months and add seven days to the first day of the last period, is a later restatement of the same 280-day interval. The rule has been tested repeatedly against observed birth timing and found slightly early. A frequently cited 1990 analysis by Mittendorf and colleagues reported that spontaneous labour in first-time mothers tended to occur closer to 288 days from the LMP, with subsequent pregnancies delivering nearer the classical figure. What changed practice more decisively was imaging. Ian Donald and colleagues in Glasgow demonstrated diagnostic obstetric ultrasound in the late 1950s, and crown-rump length charts developed in the 1970s made early biometric dating reproducible. Professional guidance now treats a first-trimester scan as the most reliable dating method available, with LMP used to set the initial estimate and ultrasound used to revise it. The trimester divisions and the milestone schedule shown here reflect that combined framework rather than Naegele's original arithmetic alone.