How Your Due Date is Calculated (The Science)
Doctors estimate your due date (EDD) to be 40 weeks from the first day of your last period. However, different scenarios require different mathematical formulas:
Naegele’s Rule (Standard)
Used for regular 28-day cycles.
Parikh’s Formula (Irregular)
Used for cycles longer than 28 days.
The "IVF" Method: Assisted reproduction dating is often more accurate because the exact conception date is known. The calculation typically calculates the EDD as Transfer Date + 266 days - Embryo Age (3 or 5 days).
The "Invisible" First Month
One of the most confusing aspects of pregnancy dating is the "first month." Many women ask:"Why am I considered 4 weeks pregnant when I only conceived 2 weeks ago?"
The answer lies int he medical standard of Gestational Age. Because ovulation is hard to pinpoint exactly, doctors start the clock from the one date you do know: the first day of your Last Menstrual Period (LMP). This means the first 2 weeks of your "pregnancy" actually occurred before you were even pregnant.
Pregnancy Milestones Timeline
| Week | Key Milestone |
|---|---|
| 4-5 | Positive Home Test |
| 6-7 | Heartbeat Visible |
| 12 | End of Trimester 1 |
| 20 | Anatomy Scan |
| 37 | Early Term |
Accuracy Reality Check
The 5% Rule: Only about 5% of babies are born on their exact Estimated Due Date.
While the calculator gives you a specific date, it is better to think of it as a "Due Month." First-trimester ultrasound is considered the "Gold Standard" for dating. If your ultrasound date differs from your LMP date by more than 7 days, your doctor will likely change your official due date to match the baby's measurements.
Frequently Asked Questions
I have a 35-day cycle. Is my due date different?
Can I calculate my due date if I don't know my LMP?
Does a 'Frozen Embryo Transfer' (FET) change the date?
Can my due date change after my 12-week scan?
Is it safe to lose weight during pregnancy?
Tools & Data Verified by the EverydayCalculators Medical Research Team.
Last updated: January 2026.