The 'leave' bit is the issue that needs resolving. Businesses should be promoting flexible working and a suitable work/life balance to ensure mothers (or fathers - I've been there) don't have to choose between work and home - the two are perfectly capable of co-existing.
I agree, however it is very hard for many businesses to do so.
Take a business that requires the management to moderate labour costs, one of the management team is a woman who has become pregnant, she decides to take her full maternity leave and she leaves for an entire year.
In this year as the company requires somebody to fulfil her role, they then need somebody to take her place... They spend a year in that role training and working hard only when that person comes back to a) be told to step down or b) the business must take the labour hit.
Both of these options are detrimental to everyone involved, i see no alternative to this situation and it can really cause huge issues in small businesses..
Then there is the other issue that the managers who haven't taken maternity leave are earning bonuses based on performance and cost control, this bonus isn't awarded to the manager on maternity because she isnt contributing to the day-to-day performance of the business.
Many people would then look at this company over a 5 year window and say "Why did your female managers earn less than the male managers?", a more "progressive" government will force the workplace to make that gap non-existant thus giving the woman on maternity leave a higher pay for less work.
There are so many situations in which the gender pay gap is just so flawed its unreal.