Due Date Calculator by Conception Date (Not LMP)

Estimating a delivery date from a known conception date rests on the observation that human gestation runs about 266 days, or 38 weeks, from fertilization. Clinicians rarely use fetal age directly; they use gestational age counted from the first day of the last menstrual period, which in a textbook 28-day cycle precedes ovulation by 14 days. That is why 266 days from conception and 280 days from the LMP resolve to the same estimated date of delivery, and why an equivalent LMP is derived here by subtracting 14 days. Gestational age is therefore always about two weeks greater than the age of the embryo itself. The 266-day figure is a median, not a deadline. A prospective study by Anne Marie Jukic and colleagues published in Human Reproduction in 2013, which used urinary hormone markers to pinpoint ovulation, found a median of 268 days from ovulation to birth with a 37-day spread across normal pregnancies. Roughly 4 percent of babies arrive on the estimated date and about 90 percent within two weeks of it. The American College of Obstetricians and Gynecologists, in Committee Opinion 700 (2017), treats first-trimester crown-rump length as the most reliable dating method, accurate to within 5 to 7 days, and recommends redating when it differs from the menstrual estimate by more than 5 days before 9 weeks or more than 7 days from 9 weeks 0 days to 13 weeks 6 days. Term is now subdivided as early term 37w0d to 38w6d, full term 39w0d to 40w6d, late term 41w0d to 41w6d, and post-term at 42w0d and beyond.

Dating a pregnancy by arithmetic goes back to Naegele's rule, which adds one year, subtracts three months and adds seven days to the first day of the last menstrual period, producing the familiar 280-day interval. The rule is named for Franz Karl Naegele, professor of obstetrics at Heidelberg, who set it out in his textbook of midwifery in the early nineteenth century. Naegele himself credited the underlying idea to the Dutch physician Herman Boerhaave, who had published a similar calculation in the previous century. The rule spread because it required no instruments and no knowledge of ovulation. Conception-based dating only became practical once ovulation could be detected. Basal body temperature charting entered clinical use in the 1930s, urinary luteinizing hormone assays became available as home tests in the 1980s, and in vitro fertilization, first successful with the birth of Louise Brown in 1978 under Patrick Steptoe and Robert Edwards, supplied a fertilization date recorded to the day. Ultrasound dating followed a parallel track: Ian Donald and colleagues in Glasgow reported the obstetric use of pulsed ultrasound in The Lancet in 1958, and Hugh Robinson published crown-rump length charts for early dating in the early 1970s. The modern vocabulary of term categories came out of the reVITALize obstetric data definitions workshop convened in 2013, which replaced the loose usage of full term for anything past 37 weeks after evidence showed measurably worse respiratory and feeding outcomes for early-term newborns.