How do they work?Gel is squeezed onto your abdomen and a transducer (or probe) is gently glided over it. The transducer emits and receives high-frequency sound waves, which are transmitted through the uterus and bounce off the baby. The images and sounds are then translated into a picture on a screen.
If you're having a scan in very early pregnancy, the sonographer may need to do a 'trans-vaginal' rather than an abdominal scan, to get an image.
The evidence so far is that ultrasounds are safe but it's recommended that its use is 'justified and limited to the minimum necessary'.
The various scans offered will depend on your local health authority. Some might not be offered in your area but will be available privately if you ask.
The different scans available
Dating scanTo accurately date your pregnancy, the sonographer will measure your baby from the top of its head to the bottom of its spine. Many women know exactly when their baby was conceived or the date of their last period, so the midwife can work out when the baby is due.
Despite this, most women still choose to have this early scan if it's offered, as they find it so exciting to see the image of their baby, with his beating heart. Also, for many, this is the moment when the pregnancy becomes real. This early scan may be when the mum-to-be finds out that she's having twins - or more!
Nuchal scanThis is done between 11 and 13 weeks of pregnancy, and a collection of fluid under the skin at the back of a baby's neck can be measured. All babies have some fluid, but in many little ones with Down's syndrome and some other conditions, the nuchal translucency is increased.
This scan is a screening test, meaning that it can estimate the risk of your baby having Down's, but it can't diagnose. If the result comes back 'high risk', you might then decide to have a diagnostic test such as amniocentesis or chorionic villus sampling.
Anomaly scanThis is offered at around 18-20 weeks and the purpose is to check that your baby is developing normally. The sonographer will look at the structure of his bones and major organs, measure him and check the position of the placenta and the amount of fluid around him.
If a problem is found or suspected, you'll be told at the time of the scan and an appointment to have another scan with a specialist should be made within 72 hours. Around 15% of scans will need to be repeated for one reason or another; most problems that need repeat scanning are not serious.
Growth scanSome women are advised to have extra growth-monitoring scans from around 28 weeks of pregnancy, if they have previously had a very small baby, are carrying twins or have high blood pressure - which can affect growth.
Group B Strep testThis is probably the least common test, but just as important. Group B Strep (GBS) is a bacterial infection that can affect a baby around labour and birth and potentially cause problems.
The test works using something called an enriched culture method (ECM). If you want it done, get your doctor or midwife to order the test. There are some NHS hospitals that do the test, but you'll probably have to pay to get it done privately.
The GBS Screening Pack is sent out by post and involves two swabs, one vaginal and one rectal. These should be taken between 35-37 weeks of pregnancy - you can do them yourself or ask your doctor or midwife to do it. The swabs are then sent straight to the testing lab in packaging that comes as part of the GBS Screening Pack. If you don't get it on the NHS, the test is usually free but you will have to pay to get the results analysed - usually around £30.
For more information about the GBS test, visit the Group B Strep Support website