Pregnancy Due Date Calculator — By Last Period, Conception or IVF
Estimate your baby's due date from your last period, conception date or IVF transfer date. See how many weeks pregnant you are, your trimester and key pregnancy milestones.
Three ways to estimate your due date
- Last period — the most common method. Enter the first day of your last period and your usual cycle length.
- Conception date — useful if you tracked ovulation or know when you conceived.
- IVF transfer date — the most precise, because the embryo's age is known exactly. Choose day-3 or day-5 transfer to match your treatment.
How the weeks are counted
Pregnancy is counted in completed weeks and days from the first day of your last period, so “12 weeks and 3 days” means twelve full weeks plus three days. It's divided into three trimesters: the first runs to the end of week 13, the second from week 14 to week 27, and the third from week 28 until birth.
What “term” means
- 37 to 38 weeks — early term.
- 39 to 40 weeks — full term, the timing with the best outcomes on average.
- 41 weeks — late term.
- 42 weeks and beyond — post-term, when doctors usually discuss inducing labour.
When the due date changes
If you have irregular periods, recently stopped hormonal contraception or aren't sure of your last period date, a date based on it can be off by a week or more. That's why most maternity services offer a dating scan between 11 and 14 weeks. Once your due date has been set from a scan, it normally isn't changed again later in the pregnancy.
Frequently asked questions
How is the due date calculated?
By Naegele's rule: 280 days (40 weeks) after the first day of your last period, adjusted if your cycle is longer or shorter than 28 days. From a conception date the calculator adds 266 days, and from an IVF transfer it adds 263 days for a day-3 embryo or 261 days for a day-5 blastocyst.
Why is pregnancy counted from my last period and not conception?
Because the date of the last period is usually known, while the exact day of conception rarely is. By convention the count starts about two weeks before conception, which is why you are already “4 weeks pregnant” when a pregnancy test first turns positive.
How likely is my baby to arrive on the due date?
Only about 4 to 5 in every 100 babies are born on their estimated due date. Most arrive in the weeks around it: between 37 and 42 weeks is considered normal, and first babies tend to come slightly later than second or third babies.
My scan gave a different due date. Which is right?
Follow the scan. A first-trimester ultrasound measures the baby directly and is the most accurate way to date a pregnancy, especially if your cycles are irregular or you're unsure of your last period. Your midwife or doctor will use it to set your official due date.
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Last updated September 25, 2026