Conception Date Calculator

Obstetric dating runs on two clocks. Gestational age is counted from the first day of the last menstrual period and totals 280 days, or 40 weeks, at term. Fetal or embryonic age is counted from fertilisation and is about 14 days shorter, giving the 266 days this calculator subtracts from a due date. The 14-day offset comes from the luteal phase, the interval between ovulation and the next period, which is comparatively fixed at 12 to 16 days across women. The follicular phase is what varies, so ovulation in a 32-day cycle falls near day 18 and in a 24-day cycle near day 10, which is why entering an actual cycle length rather than assuming 28 days changes the estimate by several days. The result is a window, not a date. Landmark fertility research established a fertile interval of roughly six days ending on the day of ovulation, produced by sperm surviving up to about five days in the reproductive tract while the released oocyte remains fertilisable for only 12 to 24 hours. Fertilisation itself cannot be observed, so even a perfectly known cycle yields a spread. Implantation follows 6 to 12 days later, mostly on days 8 to 10, which is when hCG first becomes detectable. Practical limits are worth stating plainly: only about 4 percent of babies arrive on the estimated due date, most deliver between 37 and 42 weeks, and menstrual dating assumes regular ovulatory cycles and accurate recall. First-trimester crown-rump length measurement is accurate to roughly plus or minus 5 to 7 days and supersedes menstrual dates when the two disagree by more than that margin.

The 280-day rule is named for Franz Karl Naegele, professor of obstetrics at Heidelberg, who set it out in his textbook in 1812, though the underlying observation had already been published by the Dutch physician Herman Boerhaave in 1744. Naegele's rule adds seven days to the first day of the last menstrual period and counts back three months. It became the standard of practice partly because it required no equipment and no knowledge of ovulation, which was not understood in humans at the time. Its embedded assumptions, a 28-day cycle and ovulation on day 14, were never intended as physiological claims about individual women. Everything more precise came from the twentieth century. The relatively constant luteal phase was worked out through basal body temperature and hormone studies from the 1930s onward. Obstetric ultrasound followed Ian Donald's work with engineer Tom Brown in Glasgow, published in The Lancet in 1958, and Hugh Robinson established crown-rump length as a dating measurement in the early 1970s, giving clinicians a method independent of the patient's menstrual history. Allen Wilcox and colleagues at the National Institute of Environmental Health Sciences quantified the six-day fertile window using daily hormone measurements in a prospective study published in 1995. The American College of Obstetricians and Gynecologists consolidated these into formal redating rules, most recently in its 2017 committee opinion on estimating the due date, which gives first-trimester ultrasound priority over menstrual dating.