Pregnant women in Britain routinely undergo at least one ultrasound scan. Most women are excited at the prospect of seeing their baby for the first time, and being reassured all is well. Some hospitals also offer a chance to find out the sex of the baby. Scans give useful information but they may also miss or exaggerate problems. Some midwives argue women should be given more informed choice over whether to have a scan.

What a scan shows

There are four main medical reasons for having a scan in early pregnancy. A scan gives a more accurate estimate of expected delivery date than calculating the date from the last period. Where a later date is predicted this can avoid unnecessary induction of labour. More accurate dating is also important in interpreting Down's syndrome tests. The scan will show whether the mother is carrying twins or more babies, although this is almost always detected before delivery anyway and there are usually no health benefits to early detection. Scans can show a low-lying placenta, which could cause problems in labour. But the placenta moves of its own accord in 19 out of 20 pregnancies and no further action should be needed unless other symptoms develop. Finally, the scan can indicate an abnormality with the baby. Checking this normally requires more tests. As with all screening, scans are not foolproof. Studies show that scans detect up to 80% of severe abnormalities but on rare occasions foetuses are aborted that are healthy or have insignificant problems.

Safety
There is no evidence scans cause harm to the baby but more research is needed to prove this beyond doubt.

Choice

Some midwives are concerned that scans are presented as routine rather than an option and that counselling, pointing out that abnormalities may be found, is rarely given.

Information

A leaflet, Ultrasound scans - should you have one?, published by the Midwives Information and Resource Service, is available from some midwives.

• What works? is based on reviews of the most up-to-date and reliable evidence available. It is written in collaboration with the NHS Centre for Reviews and Dissemination at York university (01904 433 634) and verified by experts.