Due Date Calculator - 無料オンライン電卓

The expected due date (EDD) is the single most-important date in pregnancy - every prenatal scan, blood test, fetal-health screening, and delivery plan revolves around it. This calculator computes EDD using Naegele's rule, the worldwide clinical standard developed by Franz Karl Naegele in 1830 and still used by virtually every obstetrician today. The formula is simple: add 280 days (40 weeks) to the first day of your last menstrual period. The calculator also adjusts for non-standard cycle lengths, giving more accurate dates for women with shorter or longer cycles.

The EDD is exactly that - estimated. Only about 5% of babies arrive on their due date; the normal range of full-term delivery is 38-42 weeks. Most first-time pregnancies tend to extend slightly past EDD; subsequent pregnancies tend to deliver closer to EDD. A first-trimester ultrasound (ideally 8-12 weeks) is more accurate than calendar-based EDD and is the gold-standard correction - once you've had that scan, the ultrasound EDD takes precedence over the calculator's estimate.

Naegele's rule adds 280 days to the first day of the last menstrual period. The 280 days break down as: roughly 14 days from LMP to ovulation (in a typical 28-day cycle), plus 266 days from conception to delivery - the actual gestational length. For non-28-day cycles, the calculator adjusts by adding (cycleLength − 28) days, because women with longer cycles ovulate later. A 32-day cycle pushes ovulation to day 18 and EDD to LMP + 284 days; a 26-day cycle moves ovulation to day 12 and EDD to LMP + 278 days.

Beyond Naegele's rule, alternative dating methods include: first-trimester ultrasound by crown-rump length measurement (most accurate, ±3-5 days), second-trimester ultrasound by biparietal diameter (less accurate, ±7-10 days), and IVF-based dating where the embryo transfer date is known precisely. For IVF, EDD = transfer date + 263 days (day-3 transfer) or + 261 days (day-5 transfer). IVF eliminates the LMP and ovulation uncertainty entirely.

Pregnancy timeline mapped against EDD: 12 weeks - end of first trimester, double-marker test; 18-22 weeks - anomaly scan; 24-28 weeks - gestational diabetes screening; 28 weeks - anti-D injection for Rh-negative mothers; 32-34 weeks - growth scan; 37 weeks - full-term begins, delivery planning; 40 weeks - EDD; 41-42 weeks - induction discussion if not delivered. Knowing the EDD lets you book all these milestones at the right time.