Most children with serious abnormalities are born into families without a history of any disorder, and hence unknown risk. 

SCREENING DIFFERS FROM DIAGNOSIS

Antenatal screening estimates the risk of having a disorder. Prenatal diagnosis establishes whether or not a disorder is definitely present in the fetus. Screening is not intended to replace diagnosis, rather it aims to provide information to help decide whether prenatal diagnosis is warranted.

RISK ASSESSMENT

Multiple markers in maternal blood and in ultrasound estimate Down’s syndrome and Edwards’ syndrome risks.  Those with high risk may consider chorionic villus sampling (CVS) or amniocentesis.   Down’s syndrome risk over 1 in 250 is regarded as high, and the screening result is said to be ‘screen-positive’ or just ‘positive’.

Gene faults found in DNA, before or during pregnancy, show that someone is a carrier.  When both partners are carriers there is a very high, 1 in 4, risk of an affected child.  Female FXS carriers are at high risk regardless of their partner.

DISORDERS

Down's syndrome  The most common cause of severe learning disability.  There are associated medical problems: but many are healthy and have a good life span.

Edwards' syndrome  Ten-times less common; nearly all die in early infancy.

DECISION MAKING

Those considering screening need to make a number of decisions. Initially there are three: whether to be screened at all, for which specific disorders, and which test. For those screened and identified as high risk, there is the further decision whether to undergo prenatal diagnosis. Following this, if the pregnancy is found to be affected, a further decision will need to be made: whether or not to have a termination of pregnancy.