Due Date

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• due_date

  Estimate a due date and gestational age from a last menstrual period, a known conception date, an IVF transfer date, or an ultrasound measurement. Use this for any pregnancy-dating question rather than doing the arithmetic. Three reasons, and they are all things models get wrong: 1. THE OFF-BY-TWO-WEEKS ERROR. Gestational age is counted from the first day of the LAST MENSTRUAL PERIOD, not from conception. At "6 weeks pregnant", conception was about 4 weeks ago. Weeks 1 and 2 precede conception entirely. This is restated wrongly constantly, and a two-week error in dating is a large error. 2. CYCLE LENGTH. Naegele's rule (LMP + 280 days) silently assumes a 28-day cycle with ovulation on day 14. For a 35-day cycle the unadjusted answer is a week out. This applies the adjustment; most calculators do not. 3. THE ACOG REDATING TABLE. Whether an ultrasound should REPLACE a period-based estimate depends on the discrepancy AND on how far along the scan was: more than 5 days up to 8w6d, 7 days