Dating scans

When will I have my dating scan?

For most Australian mums-to-be, the first scan will be a nuchal translucency (NT) scan between about 11 and 14 weeks of pregnancy (ASUM 2018, DH 2018). The NT scan is part of the combined screening test for Down syndrome, but your sonographer will also take measurements to confirm your due date.

Increasingly, though, doctors and midwives are offering women a dating scan before their NT scan. You may be offered a dating scan between eight and 10 weeks. It's not routine, but is commonly performed if you’re unsure when your baby was conceived (DH 2018).

You may be offered an earlier scan , at about six or seven weeks, if you:

• are experiencing bleeding or pain (MA nd)
• have had problems in a previous pregnancy (MA nd)
• have had recurrent miscarriages (NGA 2019)

How accurate are dating scans?

A dating scan is the best way to predict your due date (DH 2018). The purpose of the scan is to work out how many weeks pregnant you are.

You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP). But conception doesn't actually occur until around 14 days after your LMP, or later if your cycle is longer than 28 days (Mongelli 2016).

You may know when your LMP was, but not when you actually conceived. If you do know the conception date, this is counted as day 14, not day one.

If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won’t work. A dating scan is more accurate (DH 2018, Skupski et al 2016). If the scan date differs from your period dates by more than five days, the scan date will be used.

The scan will usually show that your pregnancy is less far along than suggested by your LMP.

This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue (Mongelli 2016, NCCWCH 2008). An accurate date also means your midwife or doctor will be able to better assess your baby's size, and provide the right care for your baby's gestational age, especially if you have any complications during your pregnancy. And it may help you avoid feeling disappointed or impatient towards the end of your pregnancy if you think your baby is late when they’re not.

What else will the dating scan reveal?

The scan can check that your baby’s heart is beating and they are developing normally (NHS 2017a).

The dating scan isn’t designed to look for subtle abnormalities, though. That will happen at your NT scan and 20-week (anomaly) scan. By 20 weeks, the sonographer will be able to see your baby in more detail because they’re bigger.

But some major problems can be seen at your dating scan, such as issues with the umbilical cord, or with your baby's skull or abdominal wall. You may need a second scan to check the first scan's findings.

During a dating scan, your sonographer will also check that the embryo has implanted in your uterus (womb). If it has implanted outside your uterus, it's called an ectopic pregnancy, which is a medical emergency.

The dating scan will also show if you're expecting twins, triplets or more (Mongelli 2016, NHS 2017a). It's helpful to know about multiple babies early on, and it's easier during the first trimester to see whether or not they share a placenta or have separate ones.

Finding out about having twins or more early in pregnancy also gives you more time to prepare for the birth, and for your doctor or midwife to plan your care.

What happens at a dating scan?

The dating scan usually takes about 20 minutes (NHS 2017a).

Nearly all scans after 10 weeks can be done through your belly, and most ultrasound clinics or units ask you to come with a full bladder. The sonographer will put some gel on your belly and move a small hand-held device (transducer) over your skin to get views of your baby (NHS 2017b). It won’t hurt, although you may feel a little pressure on your belly.

If your uterus is very deep in your pelvis or tilts backwards, you're less than eight weeks pregnant or you're overweight, you may be offered a vaginal scan.

A vaginal scan can get a closer view of your baby and is the most accurate dating method if you're less than eight weeks pregnant. It shouldn't hurt, but some women find it a bit uncomfortable. You don't need a full bladder for a vaginal scan.

Your sonographer may give you a report of the scan, which will tell you exactly how many weeks pregnant you are. They will send the report to your doctor or midwife, who will discuss it with you at your next antenatal appointment.

Keep your copy of the scan report or your due date with your maternity notes so your caregiver and anyone else involved in your care can see it.

How big will my baby be at the dating scan?

Your baby is measured from head to bottom. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester (ASUM 2018). How big your baby will be depends on when you have the scan:

• at 10 weeks, your baby measures about 3cm
• at 12 weeks, they measure between 5cm and 6cm
• at 13 weeks, they measure about 7cm
  (Loughna et al 2009)

After 13 weeks, your baby can curl up and stretch out, so measuring the length becomes less accurate. So from 13 weeks, if your baby is longer than 8.4cm, the circumference of their head becomes the best way to measure your baby (Mongelli 2016, NCCWCH 2008).

Can I have a screening test at the same time?

Most hospitals and ultrasound clinics offer a dating scan at the same time as the screening scan for Down syndrome and other conditions.

Screening for Down syndrome involves a nuchal translucency (NT) scan, plus a blood test. This is called the combined screening test.

You can have the NT scan from 11 weeks to 13 weeks plus six days, or when your baby’s CRL is 45mm to 84mm (ASUM 2018). It takes about 30 to 45 minutes.

As long as the timings are right, you can have the dating scan at the same time as your NT scan. But your sonographer will do the dating scan first. That’s because the screening test needs an accurate due date to be reliable (NCCWCH 2008, NHS 2017b, UK NSC 2016).

Are there any disadvantages to having a scan?

Ultrasound scans are considered to be safe for you and your baby when performed by trained professionals. But, sometimes, having a scan can cause unnecessary worry. It may show a very minor problem, or something that may get better on its own.

As with all screening tests, there can be false-positive and false-negative results. For example, the NT scan for Down syndrome has a false-positive rate of five per cent (FMF nd). This means that, out of 20 women who have the scan, one woman will be wrongly told she has a high chance of having a baby with Down syndrome.

Your doctor or midwife will discuss the benefits and disadvantages of scans with you, as well as any unusual findings from your scans. It's entirely up to you whether or not you have a scan. You could have a dating scan without the NT scan, or even change your mind on the day of your scan. Your sonographer should support your decision, either way.

More about pregnancy scans:

• Find out if it's possible to get an early scan before 10 weeks.
• Learn about private 3D and 4D scans.
• Discover whether your weight will affect your scan.
• Read more about the nuchal translucency (NT) scan.