20-week scan (anomaly scan)

Pregnant woman holding scanThe 20-week scan is also known as the mid-pregnancy ultrasound or the anomaly scan. It should be done between 18 weeks and 20 weeks plus six days gestation.

It's exactly the same format as the 12-week-scan so it's the same drill for you: full bladder before the scan, gel on belly, picture of baby on screen.

The scan may take a bit longer as the sonographer has to have a really detailed look at the development of your baby, and the health and position of your placenta.

The image of your baby is much more detailed than your 12-week scan and it can be a mind-blowing experience.

To give you a taste of what lies ahead - ie babies don't always do what you want them to - how much the scan shows may be affected by your baby's position at the time of the scan.

What does the 20-week scan show?

The anomaly scan checks your baby for structural abnormalities. It can pick up serious problems, so this is something to bear in mind if you're thinking of taking other children along.

The sonographer will check your baby's head, face, spine and see whether all the bones align.

All internal organs will be checked to see that they have developed properly. The heart is checked to ensure the four chambers are of equal size and the valves appear to be working with every heartbeat.

Your baby's kidneys and stomach will be checked to see if they're functioning properly, and limbs, hands and feet will also be examined.

The position of the placenta will be noted; if it is lying low in your uterus you will need to have another scan later on.

The sonographer will also check the umbilical cord and the volume of amniotic fluid surrounding your baby.

There will be various measurements made during the scan, mainly:

  • Head circumference
  • Abdominal circumference
  • Thigh bone length

These measurements indicate whether your baby is developing as expected and act as a double-check on your estimated due date.

As well as developmental checks, the sonographer is looking for specific conditions that may be treatable, or may jeopardise your baby's survival.

These are the different conditions that are being looked for (the varying detection rates are as documented by the NHS Fetal Anomaly Screening Programme):

  • Anencephaly (a brain disorder) 98%
  • Spina bifida 90%
  • Cleft lip 75%
  • Serious cardiac abnormalities 50%
  • Bilateral renal agenesis (where the kidneys fail to develop) 84%
  • Lethal skeletal dysplasia (where the skeleton does not develop as it should) 60%
  • Edwards' syndrome (Trisomy 18) 95%
  • Patau's syndrome (Trisomy 13) 95%

Abdominal defects:

  • Diaphragmatic hernia 60%
  • Gastroschisis 98%
  • Exomphalos 80%

If the sonographer detects any problems, you will be referred to a fetal medicine specialist and should be seen within a few days. You will be examined again and it may be that your baby can be treated within the womb or that they will need treatment after their birth.

In a few instances, you may be given the option to end your pregnancy. You will be given counselling and support when making this tremendously difficult decision.

Image: Shutterstock

Last updated: 09-Apr-2013 at 4:50 PM