How Do Pregnancy Calculators Work?
Naegele's Rule: The Basis of Every Calculation
German physician Franz Naegele set a simple rule 200 years ago that is still the standard: due date = first day of the last period + 1 year - 3 months + 7 days (that is, + 280 days). The reason: the average cycle is 28 days with ovulation on day 14, so two weeks are added before actual conception.
From Due Date to Your Current Week
We compute the difference in days between today and the LMP date, then divide by 7: the result is weeks and the remainder is days. Example: a 100-day difference gives 100 / 7 = 14 weeks with a 2-day remainder, meaning you are in week 15 + 2 days.
Hijri Conversion
We convert the Gregorian date to the Julian Day Number (JDN), then to the tabular Hijri calendar using a standard algorithm. The result is approximate, within +/- 2 days of the official Umm al-Qura calendar.
Privacy
All calculations run in your browser (JavaScript) and never leave your device; we do not send your dates to any server.