How to Calculate Your Due Date
Your estimated due date (EDD) is the day your baby is expected to arrive — about 40 weeks from the first day of your last period. Only about 1 in 20 babies actually arrives on the exact date, so treat it as a helpful estimate rather than a deadline.
Method 1: From your last period (Naegele's rule)
The classic method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). A common shortcut is: take the first day of your last period, subtract three months, and add seven days.
This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation shifts, so the due date should be adjusted by the difference — a good calculator does this for you.
Method 2: From conception date
If you know your conception (or ovulation) date, the due date is about 266 days later, since that skips the roughly two weeks between your last period and ovulation.
Method 3: From an IVF transfer
With IVF the timing is precise. For a day-5 (blastocyst) transfer the due date is about 261 days after transfer; for a day-3 embryo it's about 263 days.
Calculate it instantly
Rather than counting on a calendar, use our due date calculator. Pick your method, enter the date (and your cycle length if using your last period), and it shows your estimated due date, how far along you are in weeks and days, your trimester, and your conception date — all worked out privately in your browser.
How gestational age and trimesters work
- Gestational age is counted from your last period, not conception, so you're considered "2 weeks pregnant" at the moment of conception.
- First trimester: weeks 1–13.
- Second trimester: weeks 14–27.
- Third trimester: weeks 28–40+.
FAQ
How accurate is a due date?
It's an estimate. An early ultrasound (dating scan) is the most accurate way to confirm or adjust it, and your provider's date takes precedence.
What if I don't know my last period date?
Use the conception or IVF method, or ask your provider for a dating ultrasound.
Is this medical advice?
No — it's general information to estimate your due date. Always follow your healthcare provider's guidance.