Your estimated due date
Your pregnancy timeline
This is an estimate. Your doctor may adjust it after your first ultrasound.
How is your due date calculated?
Most due dates are counted as 280 days (40 weeks) from the first day of your last menstrual period. This is known as Naegele's rule. It assumes a 28-day cycle with ovulation around day 14, so this calculator moves the date later or earlier when your cycle is longer or shorter than 28 days.
- Last period: first day of last period + 280 days, adjusted by your cycle length minus 28.
- Conception date: conception date + 266 days.
- IVF transfer: transfer date + 266 days, minus the embryo's age (263 days for a 3-day embryo, 261 for a 5-day embryo).
- Ultrasound: scan date + 280 days, minus the gestational age measured at the scan.
How accurate is a due date?
A due date is an estimate, not a deadline. Only around 4 to 5% of babies are born on the exact day, and most arrive within two weeks either side. According to the American College of Obstetricians and Gynecologists (ACOG), an ultrasound in the first trimester is the most accurate way to date a pregnancy, so your doctor may update your due date after that scan.
Pregnancy weeks and trimesters
- First trimester: week 1 to 13 weeks and 6 days
- Second trimester: 14 weeks to 27 weeks and 6 days
- Third trimester: 28 weeks until birth
- Early term: 37 weeks to 38 weeks and 6 days
- Full term: 39 weeks to 40 weeks and 6 days
The timeline in your result shows the usual windows for the NT scan, the anomaly scan and the glucose test. Your doctor or midwife will confirm the exact dates for your care.
When to contact your doctor
Book your first pregnancy appointment soon after a positive test. Contact your doctor or go to an emergency department straight away if you have heavy bleeding, severe pain in your abdomen, a bad headache with blurred vision, fluid leaking from your vagina, or if your baby is moving less than usual later in pregnancy.
Frequently asked questions
Is pregnancy counted from conception or from my last period?
From the first day of your last period. Conception usually happens about two weeks later, so on the day you conceive you are already counted as roughly 2 weeks pregnant.
What if my periods are irregular?
The last-period method assumes regular cycles, so it is less reliable when cycles vary a lot. An early ultrasound gives a more accurate date. If you know your scan result, use the Ultrasound option above.
Can my due date change?
Yes. Many doctors adjust the due date after the first-trimester dating scan if it differs from the date based on your last period.
How is an IVF due date calculated?
Add 266 days to the transfer date, then subtract the age of the embryo in days. That is 263 days after a 3-day transfer and 261 days after a 5-day transfer.
How many babies are born on their due date?
Only a small share, roughly 4 to 5 in every 100. Most babies arrive within two weeks before or after the due date.
Is my information saved?
No. The calculation runs in your browser. Nothing you enter is sent to or stored on our servers.