Key Takeaways

1. Pregnancy dating is clinically measured from the first day of your last menstrual period (LMP), not the date of conception, which adds 2 weeks to gestational age compared to embryonic age.
2. The Naegele’s Rule formula used in most pregnancy calculators estimates your due date by adding 40 weeks, 7 days, and 9 months to your LMP start date.
3. Only 4-5% of pregnancies deliver on the exact predicted due date; 90% deliver between 37 and 42 weeks, which is considered full term.
4. First-trimester ultrasounds between 8 and 13 weeks gestation are the gold standard for adjusting due dates, with a margin of error of just 3-5 days.
5. Tracking your pregnancy week by week helps you prepare for critical prenatal screening tests, fetal development milestones, and lifestyle adjustments.
6. Due date estimates from pregnancy calculators should always be confirmed and adjusted by your licensed OB/GYN or midwife based on your personal health history.
7. If you have irregular periods, the calculator’s due date estimate will be less accurate, so you will almost always need an ultrasound to confirm gestational age.