Dating scan

A dating scan is an ultrasound done very early in pregnancy to check the baby’s development. Information from the scan is compared with the number of weeks of pregnancy and to estimate the baby’s due date of birth.

All babies are around the same size in very early pregnancy. As the pregnancy advances they grow very differently.

What exactly is a dating scan?

This is an ultrasound, done at an X-ray/ultrasound clinic or hospital. High-frequency sound waves bounce off the baby and the mother’s surrounding organs to give high density images. A dating scan is done by a sonographer with special training in scans and x-rays. The results are read by a radiographer or radiologist and the results are sent back to the health provider who ordered the scan.

Why do I need to have a dating scan?

If there’s uncertainty about when you conceived and the number of weeks pregnant you are. Many women have irregular menstrual cycles and are unsure of when they conceived. Some women conceive when they are taking the contraceptive pill or using other contraception. This makes it difficult to estimate how many weeks pregnant they are and when their baby is due.

First pregnancy scans are a very precise way of assessing a baby’s development. Dating scans are not essential and you have a choice about whether you have one or not. But they do help to give a clear idea of how the baby is developing and their due date.

A dating scan is also useful if

• There is any bleeding in the pregnancy.
• If a woman has a medical condition which could affect the pregnancy.
• If a woman has had a previous miscarriage or pregnancy complications such as an ectopic pregnancy.

How is a dating scan done?

Generally a dating scan is offered to pregnant women 10-13 weeks into their pregnancy. Previous miscarriage, pregnancy complications or fertility assistance, can mean a dating scan is done earlier than this, anywhere between 6-10 weeks of pregnancy.

A dating scan done after 10 weeks can be done via the mother’s abdomen (tummy). Until then, a vaginal ultrasound provides the best images. You will need to sign a consent form for your ultrasound and get an explanation of what’s involved. Gel will be squirted onto you tummy, this helps the transducer pick up the images more clearly.

How do I prepare for my dating scan?

If you’re having an abdominal ultrasound, you’ll need to have a full bladder. This will help to push your uterus up out of your pelvis so it will be easier to see the pregnancy sac. If you’re having a vaginal ultrasound, you won’t need to have a full bladder. Generally the scan takes around 15-30 minutes to complete.

Depending on the sonographer practice, you may get a USB or password to link into the practice website. This is so you can see and share the images of your baby if you want to.

What will I see on my dating scan?

Depending on your pregnancy weeks

• The baby’s heartbeat.
• Where the pregnancy sac is embedded in your uterus and to rule out it’s not in one of the fallopian tubes (ectopic pregnancy).
• The size and development of the baby. The baby will be measured from its head to its bottom. This is called the crown-rump length (CRL) measurement.
• Look for the baby’s movements.
• Look for one or more babies.
• Where the placenta is starting to form.
• The relationship of your uterus to other organs.
• Any abnormalities which may be visible.
• By 12 weeks the dating scan can be combined with a risk calculation for Down syndrome and other chromosomal abnormalities. Speak with your maternity care provider about this.

Generally sonographers turn one of the screens around so the mother and her partner (if present) can see the images as well.

Are there any risks to a dating scan?

• Pregnancy date scans are considered to be very safe. Sonographers are highly trained medical professionals and need to constantly update their learning.
• There can be disappointment when the dating scan does not provide the expected and hoped for information.
• They can cause worry about complications. Scans, though precise, are not always 100% accurate. They can provide false negative or false positive results. If there are concerns, a repeat scan is recommended.
• They can be uncomfortable due having a full bladder. Though this is only for a short time and worth the discomfort.

What’s next after my dating scan?

Generally a following scan is booked for between 12-14 weeks of pregnancy. It’s possible then to see the baby’s fully developed organs. Although they’re still going to be tiny, all they need to do is grow and develop towards maturity.

What else will the dating scan show?

As well as checking to see if your baby has a heartbeat, their head and limbs, trunk and body will be checked. Don’t worry if a heartbeat isn’t clear at your first dating scan. The reason for a dating scan is to determine the baby’s gestational age, not to look for abnormalities.

How big will my baby be when I have my dating scan?

Your baby is measured from head to bottom. After around week 13 the baby’s head measurement is another accurate way of measuring their growth.

• At 10 weeks, your baby will measure around 3cm.
• At 12 weeks your baby will measure between 5-6 cms.
• At 13 weeks will measure around 7cms.

FAQ

I’m allergic to latex, is that a problem?

Women with an allergy to latex can react to the probe cover if they’re having a vaginal dating scan. Let the sonographer know if you’re sensitive or allergic to latex.

When will I know the results of my dating scan?

You will need to make an appointment with your maternity care provider to get the full results of the dating scan.

Written and reviewed by Jane Barry, midwife and child health nurse on 19/04/20.