Chorionic villus sampling is an alternative to amniocentesis to test for serious foetal problems, and has the advantage that it can be done earlier, at about ten weeks after fertilisation. 

At an early stage the embryo forms itself into two parts, one becoming the future individual and the other developing into the placenta (‘afterbirth’). The part that forms the placenta starts out as finger-like sections called chorionic villi, which burrow into the wall of the womb to come close to the mother’s blood vessels. These villi are formed by division of the original fertilised egg and thus have exactly the same DNA as the embryo does, including any possible genetic abnormality. Any defect in one will be present in the other.

Chorionic villus sampling is not entirely without risk. In about 1% the procedure results in miscarriage (spontaneous abortion). There is a high natural risk of miscarriage at 10 weeks, around 2-3.

Chorionic villus sampling allows a very early diagnosis of serious genetic disorders. If the condition found can’t be treated, or if it causes severe disability in the child, the parents have the option of agreeing to termination of the pregnancy.

Many fetal disorders can be detected by chorionic villus sampling. These include:

• Down's syndrome
• Cystic fibrosis
• Thalassaemia
• Factor VIII and factor IX types of haemophilia
• Duchenne’s muscular dystrophy
• Turner's syndrome
• Sickle-cell anaemia
• Antitrypsin deficiency
• Phenylketonuria
• Mental difficulties caused by the fragile X syndrome.

Several other less common conditions caused by chromosomal abnormalities in patients in high risk groups can be diagnosed early in this way.

How is it performed

A small sample of chorionic villi is obtained by passing a thin needle either through the wall of the abdomen or through the vagina and the neck of the womb (cervix). The needle is moved to the site of the placenta under the guidance of ultrasound scanning. From the cells obtained in this way, a chromosome (genetic) analysis can be done