Dating scan

What is a dating scan?

A dating scan is an ultrasound scan done in the first trimester of pregnancy. Measurements of your baby are taken to estimate how long you have been pregnant.

A dating scan can also:

• confirm if you are pregnant with one baby or multiple babies
• check that your pregnancy is not ectopic (baby growing outside the uterus)

If you don’t know when you had your last menstrual period, a dating scan is very useful in working out when you became pregnant.

It’s important to have an accurate estimated due date (or EDD) because it helps you have the recommended tests at the right time.

Knowing how far along you are is also important if:

• your baby is born prematurely
• you haven’t given birth by your estimated due date and you might need to think about having your labour induced

Why might I need a dating scan?

Many women ovulate (release an egg that may then be fertilised) and conceive about 2 weeks after their last period. So, the baby is due about 40 weeks after the beginning of their last period. That’s why people often talk about pregnancy lasting for 40 weeks.

If you have a regular 28-day cycle, you can calculate an estimated due date for your baby. Count 40 weeks from the first day of your last menstrual period.

Working out your due date may not be so simple or accurate if:

• you have long or irregular cycles
• you don’t remember when you had your last period
• 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 usually done between 8 and 14 weeks of pregnancy (most often before 12 weeks). At this time, most babies of the same gestational age are about the same size.

If your doctor or midwife also recommends a nuchal translucency scan, they may try to arrange it between 11 and 13 weeks. This way you can have both tests at a single ultrasound scan appointment.

Do I have to have a dating scan?

Not everyone needs a dating scan. You and your doctor and midwife can decide together which tests and ultrasounds you will have during pregnancy.

You will be given information about each test so that you can make informed decisions.

Who performs a dating scan?

Your dating scan is usually performed by a specially-trained technician called a sonographer. Other health staff can also be trained to do it. This could include a doctor or midwife.

A dating scan can be done in a radiology or ultrasound clinic or at a hospital.

What happens during a dating scan?

In early pregnancy, ultrasounds (including dating scans) can be done through your abdomen (tummy) or vagina. Your sonographer will decide based on things like:

• how far along your pregnancy is
• the position of your baby
• your body shape

If your scan is done through your abdomen, you should not urinate (wee) before the scan. This makes it easier for the sonographer to see your uterus (womb).

The sonographer will apply some gel and gently move an ultrasound wand across your tummy. It shouldn’t hurt, but the gel can feel cold.

What happens during a transvaginal ultrasound scan

If your scan is done through your vagina, the sonographer will need to gently insert a small, lubricated ultrasound probe. The probe may be a little uncomfortable but isn’t usually painful.

Scans done this way can give clearer pictures because the probe is closer to your uterus. If a vaginal probe is needed the sonographer will explain what is happening first.

How should I prepare for a dating scan?

When you make the appointment ask if you need to drink water so that you have a full bladder.

Wear some loose clothes so it is easy to access your tummy for the scan.

Can a dating scan harm my baby?

Ultrasounds, including dating scans, do not harm you or your baby or increase your risk of miscarriage. No needles or radiation are used.

What do the dating scan 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 a range of when they are 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
• 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.

Can my partner come along too?

Yes. You can bring your partner, or a family member or friend to your ultrasound appointments with you. It can be useful for someone else to listen to the information provided so that you can chat about it afterwards.

Questions for your doctor

Here are some questions you might want to ask your midwife or doctor:

• Why are you offering me this test?
• What does the procedure involve, and do I need to do anything on the day?
• When will I get the results?
• Who will contact me to give me the results?
• Do I need to do anything to care for myself after the procedure?

More questions to ask your doctor about tests and scans.

ASK YOUR DOCTOR — Preparing for an appointment? Use the Question Builder for general tips on what to ask your GP or specialist.

Resources and support

Your GP, obstetrician or midwife can answer your questions and give you more information on working out your estimated due date. Ask if they recommend a dating scan for you.

Visit Inside Radiology to find out more about ultrasound and imaging scans.

You can read about pregnancy online in many languages at The Western Sydney local health district and the Royal Women’s Hospital in Melbourne.