Most pregnancy due dates are 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. For cycles that differ, the due date shifts accordingly.
How Due Dates Are Actually Determined
LMP method: Most common. Add 7 days to LMP, subtract 3 months. Example: LMP January 1 → Due date = October 8. IVF/conception method: Add 266 days to conception date if known. Ultrasound dating: An 8–10 week ultrasound measures the fetus and is often most accurate, especially with irregular cycles.
Trimester Breakdown
- •First trimester (weeks 1–12): Major organ development. Miscarriage risk highest. Morning sickness common. First prenatal visit typically at 8–10 weeks.
- •Second trimester (weeks 13–26): Often the "honeymoon trimester." Energy returns. Anatomy scan at 18–20 weeks checks fetal development. You'll likely feel first movement (quickening) at 18–22 weeks.
- •Third trimester (weeks 27–40): Rapid weight gain for baby. Braxton Hicks contractions. Group B strep test at 35–37 weeks. Baby typically positions head-down by 36 weeks.
When Babies Actually Arrive
Only 5% of babies arrive on their exact due date. 50% of births occur within 5 days before or after the due date. "Full term" is 39–40 weeks. "Early term" is 37–38 weeks. Babies born before 37 weeks are preterm. Most doctors won't induce before 41 weeks without medical indication.
Plan your maternity leave around 39 weeks, not your exact due date. Give yourself buffer — last-minute changes to your due date estimate are common as pregnancy progresses and ultrasounds are updated.