It is the first electoral test for Conservative leader Mr Cameron since he became leader last December.

BBC: NEWS | UK | UK Politics | Tories are 'satisfied' with gains

His fixing skills found their greatest test when he served as Conservative chairman for two years up to the 1997 election.

BBC: Sir Brian Mawhinney

He said the company had sought to be conservative in its rollout by limiting the test to women with risk factors and recommending that it be administered in conjunction with genetic counseling.

WSJ: New Questions on Cutting-Edge Prenatal Tests