How to use the due date calculator
- Choose how you want to calculate: from the first day of your last period (most common), your conception date if you know it, or your IVF transfer date.
- Enter the date. For the last-period method, adjust your average cycle length if it isn't 28 days.
You'll see your estimated due date, how many weeks and days pregnant you are today, your trimester, and the dates of common pregnancy milestones.
How the due date is worked out
Due date = Last period + 280 days + (cycle length − 28)
Due date = Conception date + 266 days
IVF due date = Transfer date + 261 days (5-day) or 263 days (3-day)
Example: if your last period started on January 1 and your cycle is 28 days, your estimated due date is October 8, and conception most likely happened around January 15.
Pregnancy by trimester
| Trimester | Weeks | Common milestones |
|---|---|---|
| First | 1–13 | Positive test, first prenatal visit, dating scan, first heartbeat on ultrasound |
| Second | 14–27 | Anatomy scan (18–22 weeks), first movements felt, glucose screening (24–28 weeks) |
| Third | 28–40+ | More frequent check-ups, baby gains weight, full term from 39 weeks |
What "full term" means
A baby born between 37 weeks and 38 weeks 6 days is called early term, 39 weeks to 40 weeks 6 days is full term, and 41 weeks is late term. Babies born before 37 weeks are considered preterm.
This calculator gives an estimate for general information only and is not medical advice. Your doctor or midwife will confirm your due date, usually with an ultrasound.
Frequently asked questions
How is a due date calculated?
Doctors usually use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle; if your cycle is longer or shorter, the date is adjusted by the difference. From a known conception date, the due date is 266 days (38 weeks) later.
How accurate is my due date?
It's an estimate. Only about 4% of babies arrive exactly on their due date, and most are born within two weeks either side. An early ultrasound (before 14 weeks) is the most accurate way to date a pregnancy, and your provider may adjust your due date based on it.
Why does pregnancy count from my last period, before conception?
Because the date of the last period is usually known while the exact day of conception isn't. So when you're "4 weeks pregnant", conception actually happened about 2 weeks earlier.
How do I calculate an IVF due date?
Use the transfer date and embryo age. For a 5-day (blastocyst) transfer, the due date is 261 days after transfer; for a 3-day transfer, it's 263 days after. Choose "IVF transfer date" above and pick the embryo age.
When are the trimesters?
The first trimester runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.