Question 1

What is Naegele's rule?

Accepted Answer

Naegele's rule is a method of estimating the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Developed by 19th-century German obstetrician Franz Karl Naegele, it is the standard formula used by obstetricians worldwide. It is based on an average pregnancy length of 280 days and assumes a 28-day menstrual cycle with ovulation occurring 14 days after the start of the period. About 5% of births occur exactly on the EDD, about 65% within ±1 week, and about 90% within ±2 weeks. The estimate is less accurate for cycles that are not 28 days or for IVF pregnancies, so ultrasound measurements from your obstetrician are more precise.

Question 2

How are ovulation and the fertile window calculated?

Accepted Answer

This site assumes a 14-day luteal phase. Ovulation is estimated as next period minus 14 days, and the fertile window covers 5 days before to 1 day after ovulation (7 days). Sperm can survive up to 5 days, so intercourse a few days before ovulation may also result in pregnancy. The 14-day luteal phase is a statistical average — actual length varies from 8 to 16 days. For more accurate ovulation tracking, combine with basal body temperature (BBT), LH tests, and cervical mucus observation.

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