Until people marry partners on average the same age, most couples will have a higher earning man when the woman gets pregnant, and as part time options are still rare in well-paid jobs, it usually makes sense for the man to keep that fulltime role and thus the woman is left to work round childcare.
In my case, MrNC is 7 years older so further up the career ladder etc. However he works for a rare IT firm that let him do 4 days plus some home working, though it's made clear he'll never get promoted that way. But he gets called by headhunters every few weeks telling him he could earn twice his current income with his skills (true), he says great, but tell the firm I do 4 days a week. So far over 50 firms have refused to consider talking to him, which is especially daft when employers think they couldn't afford someone with his skills, but instead of hiring 80% of him, always go for someone less well qualified.
It's got to the stage where the recruiters bet £10 they can get him interviews but then come back cursing the stupidity of the firms!
So, while he's a decent bloke and we used to have a very equal relationship, and it would be great if we could both work 3-4 days a week, doing similar amounts of childcare, jobs just aren't set up that way.