LMP method (Naegele's rule): due date = LMP + 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14. Conception method: due date = conception + 266 days (38 weeks), because ovulation usually happens about 14 days after the period. IVF methods: add 266 days minus the embryo age — 18 days for a Day-3 transfer, 19 days for a Day-5 blastocyst — because the embryo age is precisely known. That's why IVF dates are the most accurate of the three.
末次月经法（Naegele 法则）：预产期 = 末次月经 + 280 天（40 周）。此法假设 28 天周期、第 14 天排卵。受孕日法：预产期 = 受孕日 + 266 天（38 周），因为排卵通常在月经后约 14 天发生。IVF 法：在 266 天基础上减去胚胎天数——第 3 天移植减 18 天、第 5 天囊胚减 19 天——因为胚胎日龄精确已知。因此 IVF 计算是三种方法中最准确的。
