Due date calculator: how to estimate when your baby is due

Many pregnant women are impatiently awaiting the birth of their child. Comparing pregnancy weeks, ultrasound images and birth plans is a popular pastime among mums-to-be. But in order to really maximise the excitement, they first need to know their baby’s estimated due date.

How to calculate due date and pregnancy week? Why is it important to know the expected date of delivery? How accurate are due date calculators? And does cycle length matter?

Due date & pregnancy week

Choose your calculation method

First day of your last period

Day of conception

First day of your last period

Date of the first day of your last period:

Cycle length (in days):

Conception date

Conception date:

Estimated due date:

Your current week of pregnancy:

Start of your maternity leave (UK):

Please note:

The information provided by the due date calculator is for general information purposes only and is not intended as a substitute for professional medical advice or treatment. Please consult your GP or your midwife for an accurate diagnosis.

Table of content

Due date calculator: Why is it important to calculate the due date?

Pregnancy due date calculator: different calculation methods

How accurate are due date calculators?

Due date calculator: why does cycle length matter?

A final word on pregnancy due date calculators

Due date calculator: Why is it important to calculate the due date?

Knowing their baby's due date has several advantages for parents-to-be. First of all, knowing the estimated date of delivery makes it much easier to get properly excited. However, knowing what pregnancy week you’re in also has other advantages, such as:

Maternity leave planning: There are very clear rules regarding maternity leave in the United Kingdom. Statutory maternity leave can start up to 11 weeks before the expected week of childbirth and last for as long as 52 weeks in total. Female employees are required to inform their employers upfront (ideally 15 weeks before the expected due date) about their plans to take maternity leave. In order to do this, they first need to know when their baby is due. Also, they need to request statutory maternity pay, for which they also need to know their baby’s estimated due date.
Organising a baby shower: The baby shower is now a common event in the lives of mums-to-be. To make sure that the party takes place in good time before the birth, it is important to know the pregnancy week and the expected due date.
Registration at the clinic: Knowing the expected date of birth is important when it comes to preparing for birth. Only if you know the estimated date of delivery, can you inform the maternity hospital of your choice or make arrangements for a home birth.
Getting a midwife appointment: Expectant mothers who wish to be accompanied by a midwife or another healthcare professional must also know their estimated due date. This makes it easier for the midwife to plan accordingly and coordinate appointments with different clients.
Better planning with other events: Having a baby is an amazing experience, but it also represents an incision in your life that should not be underestimated. As soon as your baby arrives, everything will change. That’s why any important events or projects that are still on your mind should be completed before the birth. Again, you need to know the expected due date for this.

Schwangere Frau sitzt auf einem Teppich und hält ihren Bauch

Pregnancy due date calculator: different calculation methods

During your first appointment with your GP or midwife, you will get official confirmation of your pregnancy. As part of the appointment, your GP or midwife will also calculate your baby’s estimated due date. However, the calculation method for determining the due date can be a bit confusing.

So, what is the formula for calculating the due date? Well, there are actually two different methods, which are Naegele’s rule and due date calculation based on the date of conception.

Here is a quick explanation of the two calculation methods:

Naegele's rule: When using Naegele’s rule for calculating your baby’s due date, you need to know when the first day of your last period was. From this date, you subtract three months and then add seven days. Depending on how much your cycle length deviates from the 28-day norm, you may need to add or subtract a few days. In a last step, add one year to the calculated date to get your estimated due date.
Calculation based on conception date: With this method, you use the date of conception (which usually occurs between 12 and 24 hours after ovulation) as the starting point for the calculation. Then you subtract three months and seven days from this date before adding one year.

In addition to the expected due date, your GP or midwife will also determine which pregnancy week you are in. Knowing how far along you are in your pregnancy is important to monitor your baby’s healthy development and better understand the changes that take place in your body. The current pregnancy week is also an important reference for monitoring weight gain during pregnancy.

But how to calculate pregnancy weeks? Well, the first thing you should know is that the first week of pregnancy is not the week following the day of conception. From a medical point of view, pregnancy officially starts 14 days before the conception date, i. e. with the first day of your last period. This means that if you conceived five weeks ago, your GP or midwife will tell you that you are seven weeks pregnant.

That’s why, from a medical point of view, the first day of your last period is more relevant than the actual conception date. The standard reference period for the duration of a pregnancy, which is 280 days or 40 weeks, also starts from the first day of your last menstrual period.

How accurate are due date calculators?

In today’s digital era, a visit to the GP or an appointment with a midwife is not the only way for a pregnant woman to find out how far along she is and when her baby is due. When surfing the internet, it’s easy to find an online due date calculator or an app that can be used for calculating the estimated due date.

Pregnancy due date calculators can use two different types of data for calculating your baby’s due date. They can either calculate the due date based on the date of conception or based on the first day of the last menstrual period.

There is one question that always raises concern among mums-to-be when it comes to due date calculators, and that is how accurate they actually are. The calculation of the due date does, of course, follow clear rules and is therefore mathematically correct. Nevertheless, the calculated due date should only be seen as an estimate since there are many different factors that can influence when your baby finally arrives.

Statistics show that only around 4% of women actually give birth on the calculated due date. The large majority of babies are born within a period of 14 days before or after the calculated due date.

Schwangere Frau packt ihre Kliniktasche für die Geburt

Due date calculator: why does cycle length matter?

The length of the menstrual cycle varies, meaning that not every woman's cycle is exactly 28 days. In fact, the cycle length can be up to 38 days. Shorter cycle lengths of just 24 days are also possible. For the majority of women, the cycle length is somewhere between 25 and 35 days.

When calculating the due date using Naegele’s rule, the length of the cycle actually has a direct impact on the calculation. That’s because Naegele’s formula bases the calculation on the first day of the last menstrual period. Depending on how many days the mother’s cycle deviates from the regular cycle length of 28 days, a few days must be added or subtracted.

Due date calculators that don’t take the mother’s cycle length into account when calculating the estimated date of delivery, won’t deliver the correct estimate. In cases where the mother’s cycle length fluctuates, the due date calculation is also less accurate than usual.

However, keep in mind that the estimated due date is just that, an estimate of when your baby is likely to be born. In most cases, the actual date of delivery will be a different one.

A final word on pregnancy due date calculators

Online due date calculators are now available on many different websites. Mums-to-be can use them to calculate their estimated due date quickly and easily. Here is an overview of the most important aspects you should keep in mind.

There are two different ways of calculating the due date: Naegele’s rule and calculation based on conception date.
When applying Naegele’s rule, the starting date for the calculation is the first day of the last menstrual period.
The formula for applying Naegele’s rule is: first day of the last menstrual period + 7 days - 3 months + 1 year (assuming that the mother has a regular menstrual cycle of 28 days).
When applying Naegele’s rule, the mother’s cycle length plays an important role for the calculation. If it deviates from the standard cycle length of 28 days, this must be taken into account during the calculation.
Pregnancy weeks are also calculated starting from the first day of the last menstrual period.