The due date has been computed the same way for two centuries. Take the first day of the last period, add 280 days, and you have the estimate every obstetric chart still starts from.
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Scientific references
The rule from 1812 that still runs obstetrics
Franz Naegele formalized the calculation in the early nineteenth century, and it has survived every technological revolution since. Take the first day of the last menstrual period, add seven days, subtract three months, add a year. The arithmetic reduces to 280 days from the last period, which assumes ovulation on day 14 of a 28 day cycle. The due date it yields is really the center of a window, since only about four percent of babies arrive exactly on it, with most healthy births landing between 37 and 42 weeks.
Why weeks matter more than the date
Obstetric care runs on gestational age, the weeks since the last period, and this calculator shows where you stand today. Milestones stack up against it. The dating ultrasound lands near week 12 and refines the estimate by measuring the embryo directly, since early size tracks age almost perfectly. The detailed anatomy scan anchors week 20. Viability thresholds, genetic screening windows, and delivery planning all read off the same week counter.
When cycles break the assumption
Long or irregular cycles push ovulation later than day 14, and the standard rule overestimates how far along the pregnancy is. A 35 day cycle typically shifts the true date a week later. Early ultrasound resolves the discrepancy definitively, which is why modern guidance treats a first trimester scan as the gold standard for dating when it disagrees with the calendar by more than about a week.
Conception versus gestational age
The quirk that confuses everyone at first is that pregnancy weeks count from the last period, roughly two weeks before conception actually occurred. At the moment of conception you are already two weeks pregnant by the obstetric clock. Everyone in maternity care speaks this dialect, so the number on this page matches what your midwife and apps report.