Due date calculator

How many weeks pregnant am I?

Our pregnancy due date calculator will help you to work out when your baby’s likely to arrive, so you can start planning ahead. Most babies arrive within a week of their due date.

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Your due date is a way of predicting a period of time when you might go in to labour, based on when you conceived. It’s a useful way of monitoring your baby’s growth, either by examining your tummy or by ultrasound scan (which is more accurate as it measures size and development). It’s also a way of your midwife or doctor to accurately plan your care in terms of visits, blood tests and scans (which are important for example, to interpret early pregnancy blood tests for the risk of congenital abnormalities).

Most babies are born at full-term which is between 37 weeks (259 days) and 42 weeks (294 days), a period referred to as "term", when the baby has reached full maturity. On average, most pregnancies last around 40 weeks – which is 38 weeks after conception or 280 days.  However, no-one can predict exactly when you might go into labour and deliver your baby (or babies!). If you are expecting twins or triplets, it’s highly likely that you will deliver well before the actual ‘due’ date.

Once you know your due date, it can help you prepare and plan for your labour and birth, as well as prepare your home for your baby.

Where does "40 weeks" come from?

The most common way to calculate a due date is to set it at 40 weeks (280 days) past the woman's last menstrual period. This is known as Naegele's Rule, after the German Obstetrician Franz Karl Naegele (1778-1851) who published the method in 1806. He suggested taking the first day of the expectant mother's last period, adding one year, subtracting three months, and adding seven days, although the science behind what he said isn’t clear.

It is important to remember that your ‘due date’ is not set in stone, but is in fact an estimate of when you might go into labour – give or take 2 weeks!

How to calculate your due date

Unless you've been keeping track of when you ovulate, it’s hard to work out when you conceived, and therefore, when your baby will arrive. Our due date calculator uses the date of your last period and the average length of your menstruation cycle to calculate your due date.

Calculating due date by last period:

Calculating your due date based on the first day of your last menstrual period (LMP) works well for women who have a relatively regular menstrual cycle. But if your cycle is irregular, the LMP method may not work for you.

It relies on you remembering the date of your LMP accurately, having a regular cycle and ovulating exactly 14 days before your average menstrual cycles.

Most pregnancies last around 40 weeks (or 38 weeks from conception), so typically the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period (LMP).

Another way to do it is to subtract three months from the first day of your last period and add seven days. So if your last period started on July 10, you'd count back three months to April 10 and then add seven days, which means your due date would be April 17th.

Calculating due date by conception date:

Because the human egg is capable of fertilization for only between 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception. So if you have been tracking your ovulation, and know exactly which day you ovulated, this method should work for you. Just add 266 days to get your estimated due date.

But even if you know the exact date you had sex, the exact date of conception is hard to know. Sperm can remain alive in the female body for up to 5 days, and the egg can live for 24 hours after it’s released from the ovary. Therefore, conception can occur several days after you’ve had unprotected intercourse, so you really do need to know exactly when you ovulated as well as when you had sex that resulted in pregnancy– which is difficult.

Calculating due date for an IVF pregnancy:

In cases where the date of conception is known precisely, such as with in vitro fertilization (IVF), the EDD is calculated by adding 266 days to the date of conception if you had an embryo transfer on day .

But it depends which day you had the embryo transfer after fertilisation:

3 day blastocyst— Transfer date + 266 days (or 38 weeks) – 3 days

*Sometimes your embryos’ age is more than 3 days, so it’s important to subtract the exact age of the embryos. 

5 day  blastocyst— Transfer date + 266 days (or 38 weeks) – 5 days and so on…


How accurate are due dates?

While due dates give you a good idea of when your baby may arrive, it’s unlikely your baby will be born on this exact date. In fact, only 4% of babies arrive on their due date1 – most babies arrive within a week of their due date - around 90% of babies will arrive in the 2 weeks either before or after the predicted due date.


Will my due date change when I have a scan?

You’ll get a more accurate due date once you have your dating scan. This is usually done at around 12 weeks of pregnancy, and will be used as the final due date given to you, especially if there is more than 5-7 days difference between your date based on your last period and the scan, or if your periods were irregular or you had just stopped hormonal contraception prior to getting pregnant – as it is difficult to know when you ovulated exactly.  It will not change however if you have had IVF treatment to conceive, as the exact date of conception is then known. 

Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the LMP). It is most accurate for dating purposes between 7 and 13 weeks of pregnancy. Earlier than 7 weeks and after 14 weeks, the measurements are not as precise in terms of calculating exact gestation.

How can I plan my due date?

You can try and plan your due date – this is based around your cycle length and ovulation. You could try an online Ovulation Calculator to help you to time intercourse. But unless you’re very lucky and your periods are extremely regular, and you and your partner have no unknown factors affecting fertility, you may well not get pregnant exactly when you want to. Even if you do, your baby may have other ideas and come at a different time than predicted!

The Science Behind Calculating Your Due Date Powered by Nutricia

Ovulation and conception usually occur about two weeks after the first day of your period, and the average pregnancy lasts between 37 weeks and 42 weeks. Using this information, along with the average length of your menstruation cycle, we are able to calculate your estimated due date.

However even women who have a regular 28 day menstrual cycle might not ovulate (release an egg) on day 14 of their menstrual cycle around 20% of the time. So if there is a difference of more than 5 days or so on the ultrasound measurements compared to your last period date, the scan dating is likely to be used.

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  1. Jukic AM et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013 Oct; 28(10): 2848–2855.

Last reviewed: 16th December 2021