Decoding ultrasound scans
Understand when and why your ultrasounds are done. 

One thing that you can be sure of during your 40 weeks of pregnancy is that you will be going for numerous doctors’ appointments, and many of these will include ultrasound scans. The number of scans you will have will depend on whether you are usinga private hospital or a government one, and if you are having a high-risk or straight forward pregnancy. While seeing your little one moving around your womb is an amazing feeling, most sonar scans are for more than just getting a baby picture. Gynaecologist and obstetrician Dr Serilla Moodley breaks down what you can expect at your scans, and what they are really there for.

12- and 20-week scans

The two most important scans are the 12-week and 20-week scans. “The first trimester scan can be done between 12 weeks and 13 weeks, six days. We are quite specific about that time frame. In order for the measurements to be accurate the scans must be done at that time. We look at many features on the foetus when we do this scan, the most important being the nuchal translucency. This is a swelling at the back of the foetus’ neck which is a marker for Down syndrome, cardiac abnormalities, pre-eclampsia, and skeletal abnormalities. The value of the unchallenged translucency test is that on its own it can predict 70 percent of abnormalities, and when combined with a blood test they can predict 97 percent.”

While this 12-week scan is commonly referred to as the nuchal translucency scan, it is also used to examine other things. “We also look at other features on the foetus such as the facial angle, nasal bone, the presence of a bladder... We do cervical length and uterine artery dopplers (which measures the blood flow to the uterus and can be used to predict pre eclampsia),” Dr Moodley says.

The second screening test that is vital for you to have, whether you are at a private or government hospital, is the 20-week scan. “This can be done between 18 and 24 weeks. It is an anatomy scan of the foetus and is used to identify any structural abnormalities such as any heart valve defects, cleft lip/palate, and spinal problems. We also look at soft markers for chromosomal abnormalities,” Dr Moodley explains. “A soft marker is something unusual in a foetus, which in itself is not a problem, but when there are lots of soft markers present it can be used to predict chromosomal problems.”

If an abnormality of some kind is detected, it is important to know that these tests are all only screening tests and cannot definitively diagnose a problem. If the screening tests come back abnormal, then you will be offered diagnostic tests such as a chorionic villus sampling or amniocentesis to shed some more light on the problem, if there is one.

Overdue scan

A final important scan that you maybe ordered to have is an overdue scan. If you are still pregnant past your due date, this scan will look at the amniotic fluid volumes, placenta (for calcifications), growth and foetal weight, and the blood flow in the umbilical cord. If your caregiver is concerned at all, they will send you for a non-stress test (NST) – which measures whether your baby is under any stress in the womb.

In between your vital scans and the ones that you may choose to have, you get your routine scans. If you are going through your pregnancy in the government sector, you may only have these scans if your doctor or midwife feels that there are any complications, or if your pregnancy has been diagnosed as high-risk for any reason. Dr Moodley explains that these scans are done to follow up the growth of the foetus, to look at amniotic fluid volumes and monitor placental condition. Caregivers can use these scans to identify growth restriction in the foetus (IUGR), especially in conditions like pre eclampsia, or macrosomia in diabetics, or to monitor the condition of placenta praevia.

Just for fun

There are a few scans that are not entirely necessary, but are optional to you. The 16-week scan is mostly carried out to identify the sex of the baby (although a really experienced foetal medicine sonographer will be able to do this at the 12 week scan). The 3D or 4D scans are a nice-to have keepsake in pregnancy, as many features, especially facial features, can be identified. In proper hands they can be used to identify or confirm structural or chromosomal abnormalities. But mostly, the 4D scans are not diagnostic, and you may choose to get one done to have the images or DVD as a memento.

For your information

  • If your 12-week or 20-week scans indicate any abnormalities, you will have the option of doing either an amniocentesis or chorionic villus sampling for further diagnostic investigation.
  • An amniocentesis is done at around 16 weeks. A needle is passed through the mom’s abdomen, through the uterine wall and into the fluid that surrounds the baby. A small sample of this fluid is removed.
  • A chorionic villus sampling is usually done at around 11 weeks. A needle is passed through the uterine wall into the placenta and a small sample of the placenta is removed.
  • In both cases, the sample is sent to the genetic lab where foetal cells are cultured and the chromosomes are analysed.

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