Ultrasound scans: an overview

What is an ultrasound scan?


An ultrasound scan involves transmitting high frequency sound waves through the uterus. These bounce off the baby and the returning echoes are translated by a computer into an image on a screen that reveals the baby's position and movements. Hard tissues such as bone reflect the biggest echoes and are white in the image, and soft tissues appear grey and speckled. Fluids (such as the amniotic fluid that the baby lies in) do not reflect any echoes so appear black. It is the contrast between these different shades of white, grey and black that allows your sonographer to interpret the images.

Most parents look forward to their scan because it gives them the first glimpse of their baby. The sonographer will probably give you a printout of your baby as a keepsake, although some hospitals charge for this. However, it is important to remember that the main purpose of the scan is not to provide the first photo for your baby album, or to find out your baby's sex, either. It is to check that your baby is growing and developing normally.


Who will do the scan?


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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. Special scans may be required in some pregnancies and these will be performed by a doctor trained in ultrasound, known as a Fetal Medicine Specialist (RCOG 2000).


What is an ultrasound scan used for?


Depending on which stage of pregnancy they are done at, ultrasound scans can:

• Check your baby has a heartbeat

• Say whether you are pregnant with one baby or more

• Detect an ectopic pregnancy, where the embryo implants outside of the womb, usually in the Fallopian tube.

• Find out the cause of any bleeding you may be having

• Accurately date your pregnancy by measuring your baby

• Assess the risk of Down's syndrome by measuring fluid at the back of your baby's neck at 11-14 weeks (what's called the nuchal translucency scan).

• Find out why a blood screening test was abnormal

• Assist in performing diagnostic tests, such as CVS or amniocentesis safely by showing the position of the baby and placenta.

• Examine your baby to see if all the organs have developed normally

• Diagnose certain abnormalities, such as spina bifida

• Assess the amount of amniotic fluid you have and locate the placenta.

• Measure your baby's rate of growth over several scans.


Will the scan reveal 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 is it done?


If you're having a scan in early pregnancy read more about scans in the first trimester, you'll need to drink several glasses of water beforehand so that your uterus is pushed out of your pelvis by your bladder, allowing the sonographer to get a good picture of your baby. She will put some (usually very cold!) gel on your tummy and will move a small hand-held transducer over your skin to get views of the baby.

If your baby is still too deep in your pelvis, or if you are overweight, the images will not be very clear, so a vaginal scan may be necessary. The vaginal transducer is long and narrow to fit comfortably inside your vagina. The sonographer will use a cover similar to a condom and will lubricate this with plenty of gel so it slides in easily. It is not necessary to go in very deeply, and it will not harm you or your baby in any way. Vaginal scans give a much clearer picture of your baby, especially at a very early stage of pregnancy.


Does an ultrasound hurt?


An abdominal scan is painless except for the discomfort of the transducer pressing on your tummy if you have a very full bladder. A full bladder is not necessary for scans in later pregnancy, though some hospitals still request it. Ask your midwife what your hospital requires.

One advantage of a vaginal scan is that it is best done with an empty bladder, so many women find this scan more comfortable than an abdominal one. You may feel a bit embarrassed, but remember the sonographer is performing these scans every day. She will cover you with a sheet, and, if you relax your muscles so that the transducer can slide in easily, it should not be in the least bit uncomfortable (College of Radiographers 1998).


When are scans usually carried out?


Most hospitals offer a scan in the second trimester at about 20 weeks (the anomaly scan) to check that your baby is developing normally. Read more about scans in the second trimester.

It's now recommended that all pregnant women have a dating scan in the first trimester - ideally at 10 to 13 weeks of pregnancy - to confirm your dates (UK National Screening Committee 2003). This is especially important if you are going to have any screening tests for Down's syndrome, as knowing the exact dates makes sure your result is accurate.

If there is any need to check the viability of your pregnancy early on - if you are experiencing pain or bleeding, for example - you may have an earlier scan at about six or seven weeks. (Read more about scans in the first trimester.)

Growth scans in the third trimester (28-40 weeks) may be recommended if a previous baby was small, if you are having twins or when there are other complications of pregnancy, for instance if you are diabetic. (Read more about scans in the third trimester.)

Sometimes your midwife may suggest a growth scan if the baby feels and measures smaller or larger than expected.


Do I have to have an ultrasound?


Although the vast majority of women in the UK have at least one scan during pregnancy, it is not compulsory. Scans can give useful and reliable information about your pregnancy, and most women find them enjoyable and reassuring. However, many abnormalities cannot be seen on scan, and sometimes the scan findings can cause uncertainty and anxiety.

If your midwife recommends a scan and you'd rather not have one, ask her to discuss her reasons with you. The final decision whether or not to have a scan is entirely yours.



What if the scan shows a problem?


You will naturally be very worried if your scan suggests that there is a problem with your baby. Sometimes a definite diagnosis can be made from the scan, such as spina bifida. In other cases the scan may show minor changes, known as "markers", which may be a sign of a more serious problem, such as Down's syndrome, or may turn out to be just a variation of normal. (Read more about ultrasound markers.)

If the sonographer finds anything unusual when doing the scan, she should refer you to a doctor within 24 hours and, if it is necessary, to a Fetal Medicine Specialist within 72 hours (RCOG 2000). It may be necessary for the doctor to perform a further test such as a CVS or amniocentesis to see if the baby's chromosomes are normal.

In the unlikely event that your baby does have a serious health problem, you will need time and support to think through your choices. These may include ending the pregnancy, or preparing for the birth of a baby who needs special care or, in rare cases, carrying out surgery on your unborn baby. If you find yourself in this situation and are looking for support and information, contact the charity Antenatal Results and Choices (ARC).


Is ultrasound safe?


Ultrasound has been used in pregnancy for nearly 30 years and medical research has found no side effects. No association has been shown between ultrasound exposure and the baby's birthweight, childhood leukaemias or other cancers, eyesight, hearing or dyslexia (Salvesen 1997). Even so, most experts agree that the procedure shouldn't be done without clear medical reasons, and that all ultrasound exposure should be justified and limited to the minimum needed to make a diagnosis.