Pregnancy Ultrasound

How an ultrasound works

Ultrasound scans use sound waves to build up a picture of the inside of the body.  The ultrasound scanner has a microphone which gives off sound waves which are then reflected back and converted into an image by the ultrasound machine.

What does a 2D Ultrasound Scan look like?

With a 2D ultrasound, the Sonographer can view different layers of the baby, from the outer extremities to the internal organs.  This differs from a 3D or 4D ultrasound scan where only the outside of the baby can be seen.

When is the scan performed?

In the UK, it is common for two ultrasound scans to be performed during pregnancy; the first is usually performed when the woman first attends the antenatal clinic and the second at around 18 to 22 weeks.

Who will perform the Ultrasound Scan?

Scans are usually performed by radiographers or midwives who are specially trained in ultrasound, and are known as Sonographers.  Most have completed a post graduate Certificate, Diploma or Master's degree in Medical Ultrasound.

How is the scan performed?

Early scans (within the first 10 weeks of pregnancy) are performed by inserting a small probe into the vagina.

Later scans are performed on the surface of the abdomen.  Ultrasound gel is spread on the surface of the skin and the scanner (microphone) is passed over the surface.

Normally, the pregnant woman can view the image on a monitor and will be given a photo of the foetus.

What can Ultrasound Scans detect?

Scans within the first stage of pregnancy (before 14 weeks) are used to check how many foetuses are present and whether they are alive.  The foetus(es) can also be measured to accurately determine the birth due date.

From 18 weeks onwards, most organ systems can be examined to ensure the foetus is developing normally.

From 30 weeks onwards, ultrasound scans can estimate how well the foetus is growing.  The umbilical cord can also be examined to ensure it is functioning correctly.  The location of the placenta can also be checked. If your placenta is low-lying (across or near to the neck of the womb), you will be offered another scan later on in pregnancy to see if it has moved - the most likely outcome - in which case it won't present any further problems. If the placenta continues to block the womb, you'll be booked in for a caesarean section around your due date.

If your scan suggests there is a problem with your baby, it's important to ask how serious the problem is likely to be, whether there are better scanning techniques at your hospital, (for instance MRI), that could be used to give a more accurate diagnosis.

Will an Ultrasound Scan detect the sex of my baby?

It can do from about 18 weeks, but if your baby is lying in an awkward position it may be difficult to tell. Some hospitals have a policy of not telling women the sex of the baby, as it is not usually possible to be 100 per cent certain.

How accurate are Ultrasound Scans?

The accuracy of ultrasound scans depends upon a number of factors including:

The skill of the sonographer operating the machine

The position of the foetus

The quality of the scanning machine

If you are very overweight, the quality of the scanned images may be compromised

Although scans are a useful tool, they are not 100% definitive and may miss small problems while identifying anomalies that rectify themselves by the time your baby is born.

Are Scans dangerous to me or my baby?

Ultrasound has been used in pregnancy for nearly 30 years and there is no evidence to suggest that scans cause harm to either mother or baby.

Some people think that more long-term research needs to be done in order to make sure ultrasound is safe, but others feel it is so widely used that any problems would have shown up by now.