Health & Fitness

Pregnancy Due Date Calculator

Estimate your baby's due date from the first day of your last period, your conception date or an IVF transfer date, and see how many weeks pregnant you are today.

Free · no sign-up Runs in your browser

Calculate based on

Last period Conception date IVF transfer

First day of your last period

Average cycle length (days)

20 to 45 days. Use 28 if you are not sure.

This is an estimate, not medical advice. Only about 4–5% of babies are born on their due date. Follow the advice of your doctor or midwife.

How to use the due date calculator

Choose the information you have:

Last period: enter the first day of your last menstrual period and your average cycle length (20 to 45 days; use 28 if you are not sure).
Conception date: enter the date you conceived, if you know it.
IVF transfer: enter the embryo transfer date and whether it was a day 3 or a day 5 embryo.

Press Calculate to see your estimated due date, how far along you are today, your current trimester and the dates of key milestones. The current week is based on today's date on your device.

How the due date is calculated

Pregnancy is traditionally counted as 280 days (40 weeks) from the first day of the last menstrual period, even though conception usually happens about two weeks later. The calculator uses these rules:

Last period: due date = first day of last period + 280 days + (cycle length − 28 days). A longer cycle usually means later ovulation, so the date moves later.
Conception date: due date = conception date + 266 days.
IVF transfer: due date = transfer date + 266 days − the embryo's age in days (3 or 5).

Weeks of pregnancy are counted from an adjusted start date 280 days before the due date. The first trimester runs to 13 weeks 6 days, the second to 27 weeks 6 days, and the third lasts until birth. A pregnancy is considered full term from 39 weeks 0 days.

Worked example

If your last period started on January 1, 2026 and your cycle is 28 days long, the estimated due date is January 1 + 280 days = Thursday, October 8, 2026. The first trimester ends on April 8, the second on July 15, and full term begins on October 1. With a 35-day cycle, the due date moves seven days later, to October 15, 2026.

Good to know

A due date is an estimate, not a deadline. Only about 4–5% of babies are born on their exact due date, and most are born within about two weeks of it.
An early ultrasound scan usually gives a more reliable date, especially if your cycles are irregular. If your scan date differs, your doctor or midwife will usually go by the scan.
This calculator does not give medical advice. Always follow the guidance of your doctor, midwife or fertility clinic.

Frequently asked questions

How accurate is a due date calculator?

It gives a good estimate, but very few babies arrive on the exact date. Only about 4–5% are born on their due date. Most babies are born in the weeks around it, and a dating ultrasound in early pregnancy usually refines the estimate.

Why is pregnancy counted from the last period and not from conception?

Most people know the date of their last period but not the exact day they conceived. Counting from the first day of the last period is a long-standing medical convention, so pregnancy is said to last about 40 weeks even though the baby develops for about 38 weeks.

What if my cycle is longer or shorter than 28 days?

Ovulation usually happens about 14 days before the next period starts, so a longer cycle means later ovulation. The calculator moves the due date later by one day for each day your cycle is longer than 28 days, and earlier for a shorter cycle.

How is the due date calculated after IVF?

With IVF the age of the embryo at transfer is known. The calculator adds 266 days to the transfer date and subtracts the embryo's age, so a day 5 embryo transferred on a given date leads to a due date two days earlier than a day 3 embryo transferred on the same date. Your clinic may give you its own date, which you should follow.

Can I use the result as medical advice?

No. The result is only an estimate. Your doctor or midwife will confirm your due date, usually with an ultrasound scan, and will advise you on your care throughout the pregnancy.