According to the ESA regulations found here it's:-
Here Certain claimants to be treated as having limited capability for work-related activity 35.—(1) A claimant is to be treated as having limited capability for work-related activity if—
(a)the claimant is terminally ill;
(b)the claimant is—
(i)receiving treatment by way of intravenous, intraperitoneal or intrathecal chemotherapy; or
(ii)recovering from that treatment and the Secretary of State is satisfied that the claimant should be treated as having limited capability for work-related activity; or
(c)in the case of a woman, she is pregnant and there is a serious risk of damage to her health or to the health of her unborn child if she does not refrain from work-related activity.
(2) A claimant who does not have limited capability for work-related activity as determined in accordance with regulation 34(1) is to be treated as having limited capability for work-related activity if—
(a)the claimant suffers from some specific disease or bodily or mental disablement; and
(b)by reasons of such disease or disablement, there would be a substantial risk to the mental or physical health of any person if the claimant were found not to have limited capability for work-related activity.
