Two kids is not exactly uncommon in the UK (note - this is a guess).
If any flying female is going to have two, most companies would apperciate it to be twins of course. This is not exactly easy to arrange (Guess nr 2, but if you dont believe it - give it a try and prove me wrong).
What is then the second best?
This might be depending on the company itself and is likely decided by the details of the retraining requirements since the total time off is likely the same? wheather they come in a row or with time in between.
It is not unlikely that one (slightly bigger?) revalidation is cheaper than two.
All of this is of course depending on a job that continue to exist after the period. A company laying of staff would of course be happy to get rid of people staying home on full salary but there is regulations and they are there for a reason - anyone unhappy about the rules should spend time trying to change them or the people making them and not look down on the people affected by the rules.
(I can see that such situations when unexpected very well might wreck havoc with the balances of a small company / family operation and have all respect for people managing to deal with these kind of issues in a civilised way.)
Actually there is no problem to work while pregnant, its just the detail that you might get grounded during parts of it. If the company (management?) does chose not to use this staff for other tasks during this time, then this is the companys decision (taken either on economic/market grounds or just plain inefficient) and has nothing to do with the employee.
I might as well mention that I am from Sweden and we do have a bit different views on such issues in general than for example 411A.