How do ultrasounds work?
An ultrasound scan uses high-frequency sound waves emitted by a handheld scanner. These soundwaves ‘echo’ off the uterus to form a three-dimensional image on a monitor. Once you get used to being able to interpret what is on the screen (and depending on the stage of pregnancy and the baby’s position) it can be possible to see quite a clear image. There are no known risks to either mother or baby from ultrasounds.
Why are ultrasounds necessary, and when are they performed?
Ultrasounds have many uses that vary depending on the stage of the pregnancy. An early pregnancy scan can be performed from the six-week mark and can confirm the pregnancy, establish a due date, rule out the presence of an ectopic pregnancy, and see if there is more than one baby in the uterus. Between 11 and 13 weeks, an ultrasound can help determine the baby’s risk of having certain chromosomal abnormalities – such as Down syndrome – in combination with blood tests.
A second-trimester ultrasound is usually recommended at around 18–20 weeks to check how the baby is developing. This looks at the position of the placenta, how much amniotic fluid is surrounding the baby in the womb, the general health and growth of the baby and the development of their body parts and organs. The scan can detect problems such as limb defects, spina bifida and heart defects. It’s important to be aware that not all problems can be detected by ultrasound.
Later scans can monitor the growth of the baby and check their position before delivery. Extra scans may be recommended for multiple pregnancies, such as for twins or triplets. Ultrasounds can also give an indication of the baby’s sex, but it’s often not possible to be completely certain.
How ultrasounds are performed
Ultrasounds are usually performed while you lie down on a bed or examination table. The person performing the scan (the sonographer) will put some gel on your belly and will then slide a hand-held device across your belly while monitoring what’s being transmitted to the screen. It’s often necessary for you to drink a few glasses of water before an ultrasound as a full bladder can help position the foetus to allow for clear images.
Sometimes a vaginal ultrasound may be needed early in the pregnancy, if you are overweight, or if the baby is positioned in a way that can’t be well accessed by the abdominal scanner.
Neither type of ultrasound is painful. Some women find the feeling of pressure on a full bladder during an abdominal scan to be uncomfortable.
Further tests are sometimes needed
A report from the ultrasound will be sent to your doctor. If any foetal abnormalities are detected, further tests may be needed.Ultrasounds are optional during pregnancy, although it’s usual for at least one to be carried out so that the baby’s development can be monitored. With higher-risk pregnancies, one or more ultrasound scans may be strongly recommended. If you would prefer not to have an ultrasound, discuss this with your doctor or midwife.