Wow. There are some stone age and ignorant views on here, which of course you are perfectly entitled to hold. However, it would be rather more beneficial to equip yourselves with the facts re UK employment law before embarking on a crusade against Miss M, who is simply asking a question.
UK employment law is designed to permit a mother to have a child and then return to the same job in which she was previously employed to continue her career.
Airlines are further restricted by the medical fact that flying during the pregnancy is not particularly advisable, except during the second trimester (which I believe some airlines permit).
Logically, if a mother wants 2/3 children in quick succession, it would in fact mean that she is away from her job for a shorter time than would be the case if they were spread out, as the time off for maternity leave post-birth and time off during the pregnancy pre-birth coincide.
Thus, Miss M is in fact a greater asset to the airline, and the business retains a skilled professional once she returns.
As regards to the pay, I don't think it is that clear cut given the few numbers of female UK flight crew. I would expect that, given the grounding during the pregnancy due to medical reasons is at full pay, but maternity leave is not, you would remain on maternity leave pay during number 2 pregnancy, yet return to the full pay once that maternity period expires.
Your contract may differ, but I don't think anything is set in stone legally.
The other matter to consider is that maternity leave can be started up to 2(?) months before birth and ended earlier - if left on the lower statutory pay, that may be a way to manage a return to the flight deck and full pay more quickly.