How Due Dates Are Calculated
The standard method is 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.
If you know the exact conception date, add 266 days (38 weeks) instead, since conception typically occurs about 2 weeks after the start of the last period.
Pregnancy Timeline
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1–12 | Organ formation, heartbeat begins (week 6), morning sickness common |
| Second | 13–27 | Baby moves, gender visible on ultrasound, anatomy scan (week 20) |
| Third | 28–40 | Rapid growth, lungs mature, baby drops into position for birth |
Important Milestones
- Week 8: First prenatal appointment recommended
- Week 12: First trimester screening (nuchal translucency ultrasound)
- Week 20: Anatomy scan ultrasound — most detailed check of baby’s development
- Week 24: Viability milestone (baby has a chance of survival outside the womb)
- Week 28: Glucose tolerance test for gestational diabetes
- Week 36: Group B strep test
- Week 37: Baby is considered early term
- Week 39–40: Full term — ideal time for delivery
How Accurate Is This?
Only about 5% of babies arrive on their exact due date. About 80% are born within 2 weeks of the estimated date (between weeks 38 and 42). An early ultrasound can improve accuracy if your cycle length is irregular.
Disclaimer: This calculator provides an estimate for informational purposes only. It is not a substitute for professional medical advice. Always consult your healthcare provider for accurate pregnancy dating and care.