What is a dating scan?

A dating scan is an ultrasound scan done between 8 and 14 weeks of pregnancy to help estimate your baby’s due date. In early pregnancy, most babies of the same gestational age are about the same size. A dating scan measures your baby and this helps your doctor estimate how long you have been pregnant, and when your baby is due. Not everyone needs a dating scan, but they can be very helpful if you aren’t sure when you conceived.

It’s important to have an accurate estimated due date (or EDD) for your baby so you can have the recommended tests at the right time. Knowing how far along you are is also important if your baby is born prematurely, or if you haven’t given birth by your estimated due date and you're thinking about having your labour induced.

Who needs a dating scan?

Most babies are born about 38 weeks after conception. Since many women ovulate (release an egg that may then be fertilised) and conceive about 2 weeks after their last period, this is often about 40 weeks since the beginning of their last period. That’s why people often talk about pregnancy lasting for 40 weeks.

Women with a regular 28-day cycle can calculate an estimated due date for their baby by counting 40 weeks from the first day of their last menstrual period. This may not be so simple or accurate in other situations, like if you have long or irregular cycles, don’t remember when you had your last period, or if you got pregnant while taking contraception that affected your cycle.

If you think you may need a dating scan to help estimate when your baby is due, speak to your doctor or midwife.

When is a dating scan usually done?

A dating scan is done between 8 and 14 weeks of pregnancy (and usually between 8 and 12 weeks), when most babies of the same gestational age are about the same size. If your doctor or midwife thinks you should have a dating scan as well as a test for nuchal translucency, they may recommend you arrange it between 11 and 13 weeks, so you can have both tests during a single ultrasound scan.

Who performs a dating scan?

A dating scan is usually performed by a specially-trained technician called a sonographer, but it can also be performed by anyone who is trained to do it. This may include doctors, midwives or other health workers. It may be performed in a radiology clinic or a hospital.

What happens during a dating scan?

In early pregnancy, ultrasounds including dating scans can be done through your abdomen (tummy) or vagina. The method used will depend on a few factors, including how far along your pregnancy is and your body shape.

If your scan is being done along your abdomen (known as a 'transabdominal ultrasound'), you will be asked to drink a few cups of water before you arrive so your bladder is full. This can make it easier to see inside your uterus (womb). The sonographer will apply some gel and gently move the ultrasound probe along your abdomen. It doesn’t usually hurt.

If your scan is done through your vagina (known as a 'transvaginal ultrasound'), a small ultrasound probe is lubricated and gently inserted into your vagina. The probe may be a little uncomfortable but usually isn’t painful. Scans done this way can give more detailed pictures because the probe is closer to your uterus.

Ultrasounds, including dating scans, do not harm you or your baby or increase your risk of miscarriage.

What do the results mean?

During the scan, the sonographer will measure your baby’s length from head to bottom, known as their 'crown-rump length' (CRL). This measurement can help estimate your baby’s gestational age and when it is likely to be born.

Having an accurate estimated due date is helpful, but it’s also important to remember it’s only an estimate. Most babies are not born on their due date.

During the test, the sonographer may also:

• confirm that your pregnancy is in the right place and is not ectopic
• look for your baby’s heartbeat
• check if you are carrying more than one baby
• check that your baby’s body organs are developing normally

How much does a dating scan cost?

Medicare will cover your dating scan, but there may be out-of-pocket costs depending on where you go for the test. It’s a good idea to ask about fees when you book your appointment.

More information

Your GP, obstetrician or midwife can answer your questions and give you more information on dating your pregnancy and on whether they recommend a dating scan for you.