Preconception and antenatal screening for the fragile site on the X-chromosome

No strong evidence to show who, other than women at risk, should be screened before or during pregnancy for fragile X syndrome.

Fragile X syndrome is caused by an inherited genetic defect that can result in mental disability. There is no way of reversing the syndrome. Although it runs in families, many people do not realise they are carriers until they have an affected child. A blood test can identify whether a person is a carrier, and therefore whether a test on the unborn baby is worthwhile. Currently, only women who are thought to be more likely to pass on fragile X are offered testing before or during pregnancy. There are no trials to show whether offering the test to everyone is worthwhile.

Authors' conclusions: 

No information is available from randomised trials to indicate whether routine preconceptional or antenatal screening for fragile X carrier status confers any benefit over testing women thought to be at increased risk.

Read the full abstract...

Fragile X is the most common cause of mental retardation after Down syndrome. It is the commonest inherited cause of mental retardation, and results from a dynamic mutation in a gene on the long arm of the X chromosome. Various strategies are used for prenatal screening.


To determine whether preconceptional or antenatal screening for fragile X carrier status in apparently low-risk women confers any additional benefit over the existing practice of offering testing to women thought to be at increased risk.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (October 2008).

Selection criteria: 

Randomised clinical trials comparing women being tested regardless of family history (intervention group) with women tested only when there is a family history of either fragile X and/or other undiagnosed mental illness/impairment (control group).

Data collection and analysis: 

Three review authors independently assessed trial quality.

Main results: 

No trials were included.