How can I work out my due date?

After finding out you're pregnant, you will want to know how pregnant you are and when your baby is likely to be born. This can be done by working out your estimated due date.

You can calculate your baby’s estimated due date based on the date of the first day of your last menstrual period (LMP). Your baby will be ‘due’ around 40 weeks after the first day of your LMP. This method is particularly accurate if you have a regular menstrual cycle. For example, if you have a regular 28-day cycle, add 7 days to the date of the first day of your last period, then add 9 months.

If your cycle is longer or shorter than 28 days or irregular, working out your due date using the first day of your LMP can be more difficult and may also be more unreliable.

Another way your due date can be calculated is by using ultrasound. By measuring your baby’s size, doctors can work out how far along your pregnancy is. In early pregnancy (usually before 13 weeks) your doctor will measure the length from your baby’s head to your baby’s bottom. This is called the crown-rump length or CRL. Later in pregnancy, your doctor can measure different parts of your baby’s body, including your baby’s head, thigh bone and abdomen to work out an average size and an estimated due date.

Ultrasounds, particularly those done in your first trimester, are generally very accurate at dating your pregnancy, but sometimes there can be a difference in the due date when comparing LMP and ultrasound dates. If this happens to you, your doctor or midwife will explain which is most accurate according to your individual circumstance.

It's important to understand that your due date is only an estimate. The exact timing of your baby’s birth is unpredictable. Babies are considered ‘term’ and are usually born anywhere between 37 and 42 weeks of pregnancy. Only about 4 out of every 100 babies are born on their actual due date.

How does my menstrual cycle affect my due date?

The average menstrual cycle is around 28 days. However, the length of a menstrual cycle differs between people. Your menstrual cycle may be as short as 21 days or be as long as 35 days. If your menstrual cycle is longer, this means you ovulated later in your cycle and that your estimated due date will be later. If your menstrual period is shorter, this means that you ovulated earlier in your cycle and that your estimated due date will be earlier.

If you have an irregular cycle, meaning sometimes your menstrual cycle could be 28 days and other times 35 days, for example, it can be more difficult to accurately estimate your due date base on your LMP.

Why is it important to have an accurate due date?

It is important to have an accurate due date so that you and your chosen healthcare team can monitor your pregnancy appropriately and keep you and your baby safe. When you give birth to your baby it is important to know how far along you are in your pregnancy. Your doctors need to know whether your baby is being born early, on time or late.

If you go into labour early, it is important for your medical team to be prepared for various complications that can occur with prematurity. Your healthcare team may need to prepare to transfer your baby to a special care unit or NICU. If, on the other hand, you are overdue, your doctor or midwife may recommend you induce your labour (begin your labour) to prevent complications that can occur with being overdue. This will be done in consultation with you, taking into consideration your birthing preferences.

Can my due date change?

If you have a regular menstrual cycle and have used this to work out your due date, or if you have had an early dating ultrasound your estimated due date is unlikely to change later in pregnancy. Ultrasounds done later in pregnancy are not as accurate at dating your pregnancy due to variations in babies’ growth and development. For this reason, your due date calculated earlier in pregnancy will be most accurate and is unlikely to change.

If you haven’t had an early dating ultrasound, a second or third trimester ultrasound can be used to date your pregnancy.

If your due date has been changed, speak to your doctor or midwife to understand why.

When should I see my doctor?

Usually, you can work out your due date without seeing a doctor. Choosing to have an early dating scan, although recommended, is your choice.

However, there are a few situations when having an ultrasound scan early in pregnancy may be strongly recommended. You should see a doctor if you:

• have a very long or irregular menstrual cycle or don’t know your LMP
• became pregnant while taking the oral contraceptive pill
• have any bleeding during your pregnancy
• have a medical condition or illness that could affect your pregnancy
• have had a previous miscarriage or ectopic pregnancy

Resources and support

Use the Pregnancy, Birth and Baby calculator to estimate your due date.