Working out your due date
9-minute read
Key facts
- Your due date is an estimation of when your baby may be born.
- You can calculate your baby’s due date using the date of the first day of your last menstrual period (LMP) or by ultrasound.
- Pregnancy generally lasts about 40 weeks, with most babies born between 37 and 42 weeks of pregnancy.
- An accurate due date helps you and your healthcare team monitor your pregnancy and baby’s growth to help keep you both safe.
- Your doctor or midwife will help you work out your due date at your first antenatal appointment.
How can I work out my due date?
After finding out you're pregnant, you will want to know how far along you are in your pregnancy and when your baby is likely to be born. This can be done by working out your estimated due date (EDD).
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 ‘full term’ if they are born at any time between 39 and 41 weeks of pregnancy. Only about 5 out of every 100 babies are born on their actual due date.
Calculating your due date using your last menstrual period
You can calculate your baby’s estimated due date using the date of the first day of your last menstrual period (LMP). Your baby is ‘due’ around 40 weeks after the first day of your LMP. This method is most 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.
Calculating your due date using ultrasound
Another way your due date can be calculated is with an ultrasound scan. Your doctor or sonographer (ultrasound technician) can work out how far along your pregnancy is by measuring your baby’s size.
During an ultrasound scan in early pregnancy (usually before 13 weeks), your doctor or sonographer measure the length from your baby’s head to your baby’s bottom. This is called the crown-rump length (CRL). This measurement is compared to population data to calculate your baby’s estimated due date.
Later in pregnancy, your doctor can measure different parts of your baby’s body, including your baby’s head, thigh bone and abdomen. These measurements help estimate the baby’s size and an estimated due date.
Ultrasound scans, 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.
How does my menstrual cycle affect my due date?
The average menstrual cycle is around 28 days, but the cycles can vary widely 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 (for example, 21 days one month and 35 days the next), 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?
Having an accurate due date helps your healthcare team monitor your pregnancy appropriately, and keep you and your baby safe.
Some pregnancy conditions affect different stages of pregnancy. Knowing at what stage of pregnancy you are can help your doctor or midwife assess for and treat any complications.
Also, knowing your baby’s gestational age helps your doctor or midwife provide the best care. When you go into labour, knowing your due date can help you and your health team know whether your baby is early or late, and whether they are at risk of any complications.
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 you are overdue, your doctor or midwife may recommend inducing (beginning) your labour to prevent complications that can occur with being overdue. You can discuss the risks and benefits of any intervention with your doctor or midwife and make an informed decision.
Can my due date change?
If you have a regular menstrual cycle and have used your LMP 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.
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.
Do I need to see a doctor to confirm my due date?
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 an ultrasound scan in early 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
When should I see my doctor?
It’s best to have your first antenatal visit before 10 weeks of pregnancy. It’s a good idea to schedule this visit for when you are about 6 to 8 weeks pregnant, so you can learn more about caring for your health during pregnancy and prevent any late complications. You can also use this visit to discuss antenatal testing with your doctor or midwife.
Your first appointment may be with a midwife or your doctor. It may be at a clinic or hospital — based on your choice.
Learn more about what will happen at your first antenatal visit.
Resources and support
You can use the Pregnancy, Birth and Baby calculator to estimate your due date based on your LMP.
Speak to your doctor, midwife or obstetrician if you have questions about your pregnancy.
Pregnancy, Birth and Baby also has more information on:
- Maternity care in Australia
- Having a healthy pregnancy
- Anatomy of pregnancy and birth
- Checkups, screenings, scans and tests you can have during your pregnancy
- Common conditions you might experience during your pregnancy
- Looking after your mental health and wellbeing during pregnancy
Speak to a maternal child health nurse
Call Pregnancy, Birth and Baby to speak to a maternal child health nurse on 1800 882 436 or video call. Available 7am to midnight (AET), 7 days a week.