Ultrasound scans use sound waves to create a picture of the baby in the womb. The main purpose of pregnancy ultrasound scans is to see whether the pregnancy and the baby’s growth and development are progressing normally.
- Early in pregnancy, ultrasound is used to check your estimated due date, the number of babies and the baby’s development.
- Later in pregnancy, the growth and position of the baby and the position of the placenta can be checked using ultrasound.
- In general, most (96 to 98 out of 100) pregnant women give birth to a healthy child. But ultrasound sometimes detects abnormalities.
- If abnormalities are detected, you may be offered more invasive tests, such as amniocentesis, to confirm the diagnosis.
- Many parents find ultrasound reassuring, but it is important to realise that these scans are a diagnostic procedure. The results they produce may be anxiety provoking – and may require some difficult decisions as well.
What ultrasound scans are offered during pregnancy?
Most women have one or more ultrasound scans during pregnancy. The two scans routinely offered during pregnancy are the nuchal scan and the anatomy scan. Sometimes other scans are performed earlier or later in pregnancy to check the growth of the baby, the position of the baby or the placenta, or for other reasons.
The four most common stages to have a scan are:
You may be offered a scan in the first 14 weeks if you are not sure when you had your last period, or if you have irregular periods.
A scan may also be arranged if you have any bleeding in the first trimester of pregnancy to make sure the baby is okay. A baby and heartbeat can be seen on the scan after 6 weeks. Before 6 weeks it is not always possible to see the baby as it is too small.
Nuchal translucency scan
You will be offered a scan between 11 and 14 weeks' gestation. This is part of first trimester combined screening for Down syndrome and other conditions (read more below).
The scan result is used to calculate the chance of a baby having a problem, such as Down syndrome, but the scan will check for twins and is also a very good way of calculating when a baby is due.
Although most of the baby’s organs have developed by 14 weeks, the baby is still quite small, about 5–8 cm long, so it is difficult to see if the baby’s development is normal at this stage.
You will be offered a scan at around 18–20 weeks’ gestation, which is also called the second trimester scan.
Many important developmental problems can be seen with a scan at this stage. This scan is usually the most detailed examination and includes assessment of the development of the baby and the position of the placenta.
Placenta previa (where the placenta is covering the cervix) can be diagnosed during the anatomy scan, but as the baby develops and the uterus gets bigger, the placenta usually moves away from the cervix. It is usually not possible to know if the placenta has moved far enough for a normal birth until 32 weeks and sometimes even later. You may be offered additional scans at this stage.
You may be offered a scan at any time after 20 weeks if your maternity care provider is concerned about the baby’s growth.
Not all babies are born the same size, but some babies are smaller than they should be. These babies can have problems during labour and sometimes need to be born early. Other babies are too big, especially if the mother has diabetes.
Scans are good for checking the size of a baby but they only give an estimate of the baby’s weight. To check the growth of the baby it is necessary to do two scans a couple of weeks or more apart. You need to wait a couple of weeks between scans because babies do not grow enough in only a few days to be able to accurately measure the difference.
Remember – no test is perfect!
There are several important things to remember regarding ultrasound scans.
- Not all problems cause a change in the anatomy of a baby, so they cannot be seen with a scan.
- Sometimes the changes are very difficult to see, especially if there are twins or the baby is in a difficult position, or if the mum is very overweight.
- Occasionally something is seen on the scan that might indicate there is a problem, but usually, the baby is fine. This situation can be very difficult because it obviously causes the parents and family to worry about their baby.
- If any problem is found on the ultrasound scan you will usually be referred to an obstetrician who will repeat the scan, tell you about the problem and explain what your options are.
What happens during an ultrasound scan?
A pregnancy ultrasound is normally carried out by specially trained staff called sonographers. You will be asked to lie on the examination table and will need to lift your top to your chest and lower your skirt or trousers to the top of your hips, to expose your stomach.
The sonographer will put ultrasound gel on your stomach to ensure good contact between your skin and the machine. They will then pass a probe over your skin. This sends out ultrasound waves and picks them up again when they bounce back. A black and white picture of your baby will be shown on a small screen.
Having the scan does not hurt, but the sonographer may have to push quite firmly at times in order to see the deeper structures.
The scan usually takes about 20 to 30 minutes.
Can an ultrasound scan harm me or my baby?
Ultrasound scans are painless and non-invasive. There are no known risks to the baby or the mother from having an ultrasound scan. However, ultrasound energy has the potential to produce biological effects on the body and therefore should be only be performed by appropriately trained healthcare providers for medical purposes (see the table above). Additional keepsake scans solely for the purpose of producing an image or video are not recommended.
If you choose to have a scan, you also need to be prepared for the possibility that the information it reveals about your baby may not be reassuring. You may be put in a position where you need to decide whether to have more invasive tests, such as amniocentesis, which increases your risk of miscarriage. If abnormalities are confirmed, the question of whether or not to continue the pregnancy might arise. Making these decisions can be unsettling and challenging.
Can I opt out of ultrasound scans?
The ultrasound scans you will be offered in your pregnancy are optional. You are well within your rights to decline any or all scans in pregnancy – or to agree to them all.
Ultrasound scanning has clear benefits when performed for medically necessary reasons. However, routine scanning of women whose pregnancies are progressing normally does not lead to healthier babies or fewer problems during labour and birth.
- Pregnancy and birth: Ultrasound scans in pregnancy Informed health online: current medical knowledge, Institute for Quality and Efficiency in Health Care (IQWiG, Germany)
- Routine ultrasound in late pregnancy (after 24 weeks' gestation) to assess the effects on the infant and maternal outcomes Cochrane Review
- Routine compared with selective ultrasound in early pregnancy Cochrane Review
- Ultrasound imaging FDA, 2018
|Dr Jeremy Tuohy is an Obstetrician and Gynaecologist with a special interest in Maternal and Fetal Medicine. Jeremy has been a lecturer at the University of Otago, Clinical leader of Ultrasound and Maternal and Fetal Medicine at Capital and Coast DHB, and has practiced as a private obstetrician. He is currently completing his PhD in Obstetric Medicine at the Liggins Institute, University of Auckland.|