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
All women, whatever their age, have a chance of delivering a baby with a chromosomal abnormality such as Down's syndrome. The chance of the fetus having a chromosomal abnormality depends on the age of the mother and also on the amount of fluid behind the neck of the fetus (nuchal translucency).
