{"id":23357,"date":"2026-10-01T02:09:04","date_gmt":"2026-10-01T02:09:04","guid":{"rendered":"https:\/\/www.youragecalculator.com\/blog\/?p=23357"},"modified":"2026-10-01T02:09:04","modified_gmt":"2026-10-01T02:09:04","slug":"pregnancy-due-date-calculation-math","status":"publish","type":"post","link":"https:\/\/www.youragecalculator.com\/blog\/pregnancy-due-date-calculation-math\/","title":{"rendered":"The Math Behind a Due Date: Why 40 Weeks Is Not 9 Months"},"content":{"rendered":"
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A pregnancy is dated as 40 weeks, which is 280 days \u2014 about 9 months and a week. Yet almost nobody delivers exactly on the estimated due date. That gap between the arithmetic and the outcome confuses a lot of people, and it is entirely explainable. Here is the actual calculation clinicians use, why it is only a midpoint, and how to run the numbers yourself.<\/p>\n

Naegele’s Rule: The Standard 280-Day Formula<\/h2>\n

The estimate clinicians call the EDD (estimated date of delivery) comes from Naegele’s rule, published in the early 1800s and still the default:<\/p>\n

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  1. Take the first day of the last menstrual period (LMP).<\/li>\n
  2. Add 7 days.<\/li>\n
  3. Subtract 3 months.<\/li>\n
  4. Add 1 year.<\/li>\n<\/ol>\n

    Worked example: LMP = June 10, 2026. Add 7 days \u2192 June 17. Subtract 3 months \u2192 March 17, 2027. Add 1 year is already satisfied. EDD = March 17, 2027. That is 280 days from the LMP.<\/p>\n

    Why 280 Days Is a Convention, Not a Measurement<\/h2>\n

    Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Those assumptions fail for a large share of people:<\/p>\n\n\n\n\n\n\n\n
    Assumption<\/th>\nReality<\/th>\nEffect on EDD<\/th>\n<\/tr>\n<\/thead>\n
    28-day cycle<\/td>\nNormal range 21-35 days<\/td>\nShifts by up to \u00b17 days<\/td>\n<\/tr>\n
    Ovulation day 14<\/td>\nDay 11-21 common<\/td>\nShifts by up to \u00b17 days<\/td>\n<\/tr>\n
    Regular cycles<\/td>\nIrregular cycles common<\/td>\nWide uncertainty<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    This is why a first-trimester ultrasound is more accurate than the LMP date. Before 14 weeks, crown-rump length predicts gestational age to within about 5 days. After that, the margin widens, so clinicians keep the early scan date rather than revising later.<\/p>\n

    Gestational Age vs Fetal Age \u2014 Two Different Numbers<\/h2>\n

    Here is a nuance that trips up almost everyone reading a scan report:<\/p>\n