The fetus can be seen by ultrasound examination which is carried out through your abdomen. Occasionally the view is not clear and it may be necessary to perform a vaginal scan as well.

It is not necessary to have a full bladder for the scans.

The aims of the scan are:

• To date the pregnancy accurately

This is particularly relevant for women who cannot recall the date of their last period, have an irregular cycle, or who have conceived while breast-feeding or soon after stopping the pill. By measuring of the crown-rump length (CRL) of the fetus, the gestational age and therefore the expected date of delivery can be accurately calculated.

• To diagnose multiple pregnancy

Approximately two per cent of natural conceptions and 10 per cent of assisted conceptions result in multiple pregnancy. Ultrasound scanning can identify if twin babies are sharing the same placenta which can lead to problems in the pregnancy. In such cases it would be advisable to monitor the pregnancy more closely.

• To diagnose early pregnancy failure

Unfortunately, in about three per cent of women who attend for an 11 to 14 week scan it is found that the fetus has died, often several weeks before and without any warning. Couples will receive full counselling as to the possible causes of this problem and the options for subsequent measures that may be necessary.

• To diagnose major fetal abnormalities

In about one per cent of pregnancies the fetus has a major abnormality that may be visible at the 11 to 14 week scan. These abnormalities can be incompatible with life or they could lead to serious handicap, but in some cases they may be corrected by surgery before or after birth. If an abnormality is detected the significance of the findings will be discussed with the parents.

• To assess the chance of Down's syndrome and other chromosomal abnormalities

The chance that the fetus has Down's syndrome at 12 weeks is higher than at birth because some fetuses with this condition die during pregnancy.

In 95 per cent of fetuses the nuchal translucency is low and the chance that the fetus has a chromosomal abnormality is decreased. In five per cent of pregnancies the nuchal translucency is high. In these cases the chance the fetus has a chromosomal or other abnormality is higher than average but it is important to realise that a majority of fetuses will be normal.

The only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as chorion villus sampling (CVS) or amniocentesis. However these tests carry a risk of miscarriage of about one per cent.

After the scan the estimated chance for Down's syndrome will be discussed with you. It is up to you and your partner to decide whether or not the chance of the fetus having a chromosomal abnormality is high enough to warrant having an invasive test such as CVS.

Irrespective of whether or not you decide to have an invasive test, it is recommended that you have a scan at 20 weeks to check for physical abnormalities. This is particularly important in fetuses with a high nuchal translucency because these fetuses have an increased chance of cardiac and other abnormalities.

• During the scan we will measure the size of the fetus, look for any major abnormalities and measure the nuchal translucency.

• Some women do not want to be told if the fetus has an abnormality or a high nuchal translucency. If this is the case your wishes will be respected but please let us know before the scan is carried out .