20-week ultrasound (anomaly scan)

What is an anomaly scan?

An anomaly scan, also known as a mid-pregnancy or fetal morphology scan, takes a close look at your baby's size and anatomy, as well as your uterus (womb).

The person carrying out the scan (sonographer) is a highly specialised radiographer with expertise in ultrasound during pregnancy. Your sonographer will check that your baby is developing normally, and look at where the placenta is lying.

Although the anomaly scan is often called a 20-week scan, you may have it any time between 18 and 22 weeks, although it’s usually done between 18 and 20 weeks (DH 2018, NCCWCH 2008, PHE 2020, RANZCOG 2015).

This image shows a baby's face and hands at 20 weeks, and gives you an idea of what you’ll be able to see at this scan.

Seeing your baby on a screen can be really exciting. You can also take your partner, a friend or a family member along to share the experience with you.

If you have a baby or young child, try to arrange for someone to babysit them at home. If that's not possible, your partner or other support person may need to take your child for a walk outside if your child gets bored or they’re distracting you or the sonographer.

Do I have to have an anomaly scan?

It's up to you. Early in your pregnancy, your doctor or midwife should explain why the scan is being offered, how it will help, and what it won't be able to tell you (RANZCOG 2018).

This will allow you time to decide whether or not you’d like to have the scan.

What will I be able to see on the 20-week scan?

Most hospitals and ultrasound clinics allow you to watch the scan, which takes about 30 to 45 minutes, as it's being carried out (PHE 2020).

If you haven't already had a scan in your pregnancy, your sonographer will check that there's only one baby, and confirm your due date.

The sonographer will point out your baby's heartbeat and parts of the body, such as the face and hands, before looking at your baby in detail. It may be hard for you to make out your baby's organs, because your sonographer will look at them in cross section.

Your baby's bones will appear white on the scan, and soft tissue such as organs will look grey and speckled. The amniotic fluid surrounding your baby will look black.

After you've seen your baby on the screen, some sonographers will turn the screen away for the rest of the scan, and show you views at the end. Some hospitals and clinics have a second monitor above, or at the foot of the bed, so you can watch the entire scan.

Will I be able to find out the sex of my baby at my 20-week scan?

The main purpose of the scan is to check that your baby is developing normally, rather than whether you're expecting a boy or girl. But you may want to know the sex of your baby.

Sometimes the sonographer can’t get a good enough view to tell your baby's sex, perhaps because of your baby’s position, or if you have a lot of tummy fat (NHS 2018).

If you don't want to find out the sex of your baby, make sure you tell your sonographer at the beginning of the scan.

Can I have a photo of my 20-week scan?

Most hospitals and clinics will offer printed photographs or a CD/DVD of images from your scan. They'll often charge you for these, so you may want to ask about your options when you make your booking. Some private ultrasound clinics now offer to upload images to a free smartphone app.

Photos are usually printed on thermal paper, which is heat-sensitive. So don’t laminate them.

What will the sonographer look at on my 20-week scan?

As well as examining all your baby's organs and taking measurements, your sonographer will look at:

• The shape and structure of your baby's head and brain. At this stage, severe brain problems, which happen very rarely, are visible.
• Your baby's face, to check for a cleft lip. Cleft palates inside a baby's mouth are hard to see and aren't often picked up.
• Your baby's spine, both along its length, and in cross section, to make sure that all the bones align, and that the skin covers the spine at the back.
• Your baby's abdominal wall, to make sure it covers all the internal organs at the front.
• Your baby's heart. The top two chambers (atria) and the bottom two chambers (ventricles) should be equal in size. The valves should open and close with each heartbeat. Your sonographer will also examine the major veins and arteries that carry blood to and from your baby's heart.
• Your baby's stomach. Your baby swallows some of the amniotic fluid that they lie in, which is seen in the stomach as a black bubble.
• Your baby's kidneys. Your sonographer will check that your baby has two kidneys, and that urine flows freely into the bladder. If your baby's bladder is empty, it should fill up during the scan and be easy to see. Your baby has been doing a wee every hour or so for some months now!
• Your baby's arms, legs, hands and feet. Your sonographer will look at your baby's fingers and toes.
• The placenta, umbilical cord and amniotic fluid.
  (ASUM 2018, PHE 2020)

The placenta may be on the front wall (anterior) or the back wall (posterior) of your uterus, usually near the top (fundus). If the placenta is near the top, it may be described as fundal on your scan notes.

The placenta will be described as low if it reaches down to or covers your cervix (the neck of your uterus). If the placenta is lying low in your uterus, you'll have another scan in the third trimester to check its position. By then, it's likely the placenta will have moved away from your cervix.

Your sonographer will check to see that there's enough amniotic fluid for your baby to move freely. It's also possible to count the three blood vessels (two arteries and a single vein) in the umbilical cord (ASUM 2018).

During the scan, your sonographer will measure parts of your baby's body, to see how well your baby is growing. Your sonographer will measure your baby's:

• head circumference (HC)
• abdominal circumference (AC)
• thigh bone (femur) (FL)
  (ASUM 2018)

The measurements should match up to what's expected for your baby, depending on when your due date is. The due date will have been estimated at your nuchal translucency (NT) or dating scan. If your anomaly scan is the first scan you've had, it'll be used to work out your due date.

Which abnormalities can be seen on the 20-week scan?

Sonographers have a list of conditions to look out for (PHE 2020). Some conditions are treatable once your baby is born. Rarely, though, a condition may be so serious that a baby can't survive.

If the condition is treatable, it will help your hospital team to know in advance, so they can make sure your baby has the right care as soon as they’re born.

Some conditions are easier to spot than others, and some are hard to see at all. Most of the conditions on your sonographer’s list are very rare. Here's the list of conditions, and the percentage chance of your sonographer seeing each one, if your baby has it:

• absence of the top of the head (anencephaly): 98 per cent
• cleft lip: 75 per cent
• defect of the abdominal wall, where the bowel and liver protrude (exomphalos): 80 per cent
• defect of the abdominal wall, where the intestines protrude (gastroschisis): 98 per cent
• missing or very short limbs (lethal skeletal dysplasia): 60 per cent
• defect of the spinal cord (spina bifida): 90 per cent
• major kidney problems (missing or abnormal kidneys): 84 per cent
• hole in the muscle separating chest and abdomen (diaphragmatic hernia): 60 per cent
• Edwards syndrome or Patau syndrome (chromosomal abnormalities): 95 per cent
• major heart problems (defects of chambers, valves or vessels): 50 per cent
  (PHE 2018)

Having an anomaly scan will most likely rule out all these conditions, as the vast majority of babies are born healthy (RANZCOG 2018). Some conditions, though, including heart defects and bowel obstructions, may not be seen until later in your pregnancy.

What if there are signs of a problem on my 20-week scan?

About 15 per cent of scans will need to be repeated for one reason or another. Most reasons that require a scan to be done again aren't serious, though.

The most common reason is that the sonographer hasn't seen everything they need to see. This may be because your baby isn't lying in a good position, or that you're overweight, in which case the scan should be repeated at 23 weeks (PHE 2015).

If your sonographer finds or suspects a problem, they or another senior staff member at the clinic may discuss it with you. It's more likely, though, that they'll call your doctor or midwife shortly after your scan and provide a report. You'll then have a follow-up appointment with your doctor or midwife to discuss the findings. If necessary, your caregiver will refer you urgently to a fetal medicine specialist.

If the specialist suspects that your baby has a heart problem, you'll be asked to come in for a fetal echo scan. The fetal echo scan will take a detailed look at your baby's heart.

Although there won't be anything of concern most of the time, abnormal findings are discovered in a small number of cases. Not all abnormal findings are serious, but it's helpful to be aware of the possibility of an abnormal finding when you're preparing for the scan.

If any scan does reveal a serious problem, you should be given plenty of support to guide you through all the options (RANZCOG 2018).

Other problems may mean that your baby needs surgery or treatment after birth, or even surgery while still in your uterus (womb). There will be a whole range of people to support you through this, including midwives, obstetricians, paediatricians and physiotherapists.

Read more about pregnancy ultrasound scans.