Calculate pregnancy due date and milestones
Due date is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. Cycle length adjustments improve accuracy.
Due date calculations are estimates—only about 5% of babies arrive on their exact due date. Most babies are born within 2 weeks before or after. Early ultrasound dating (8-12 weeks) is often more accurate than LMP calculations.
First trimester: weeks 1-12 (organ development, highest miscarriage risk). Second trimester: weeks 13-26 (often called the 'golden period,' baby movements begin). Third trimester: weeks 27-40 (rapid growth, preparation for birth).
Full term is 39-40 weeks. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42+ weeks. Babies born at 37+ weeks are considered term, but 39+ weeks is optimal for development.
Gestational age counts from the first day of your last period (even though conception occurs ~2 weeks later). Fetal age (or conceptional age) counts from actual conception. Gestational age is 2 weeks longer and is the standard medical measurement.
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