Most common method: add 280 days (40 weeks) to first day of last menstrual period (LMP). This is Naegele's rule. Conception date method: add 266 days (38 weeks).
Why 40 weeks if pregnancy is 9 months?▼
Pregnancy is measured from LMP, which is ~2 weeks before conception. So 40 weeks = ~9 calendar months (38 weeks from conception).
How accurate are due dates?▼
Only ~5% of babies arrive on their due date. Most arrive within 2 weeks before or after. First trimester ultrasound is most accurate.
What are the trimesters?▼
1st: Weeks 1-13, 2nd: Weeks 14-27, 3rd: Weeks 28-40+. Each trimester has specific developmental milestones and prenatal care.
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