How due dates are calculated
Doctors usually count pregnancy as 40 weeks (280 days) from the first day of your last menstrual period (Naegele’s rule). If your cycle is longer or shorter than 28 days, the date shifts by the difference. From a known conception date, add 266 days. For IVF, add 261 days to a day-5 transfer or 263 days to a day-3 transfer.
Pregnancy at a glance
| Stage | Weeks |
|---|---|
| First trimester | Weeks 1–13 |
| Second trimester | Weeks 14–27 |
| Third trimester | Weeks 28–40+ |
| Early term | 37w0d – 38w6d |
| Full term | 39w0d – 40w6d |
Only a small share of babies arrive exactly on their due date — most are born within about two weeks either side. Your provider may adjust your due date after an early ultrasound, which is the most accurate method.
Sources & review
Checked against these sources on November 10, 2026. Rules and rates can change — always confirm with the official source.
Due Date Calculator FAQs
How accurate is a due date calculator?
It gives the standard estimate used by clinicians. An early ultrasound (before about 14 weeks) is more accurate and may change your date.
What if I do not know my last period date?
Use a conception or IVF date if known, or ask your provider about a dating ultrasound.
Is my information stored?
No. Everything stays in your browser.