Equality Act 2010 s.18(6) defines what is called "the protected period". This in relation to a woman's pregnancy is defined as the period which:

"begins when the pregnancy begins, and ends -

(a) if she has the right to [ordinary maternity leave] and [additional maternity leave], at the end of the additional maternity leave period or (if earlier) when she returns to work after the pregnancy;

(b) if she does not have that right, at the end of the period of 2 weeks beginning with the end of the pregnancy"

For relevant general notes see Equality Act 2010/protected characteristics/pregnancy and maternity