Back in the (not so good) old days men worked outside the house and women worked inside the house (aha you won't catch me saying men worked and women didn't!)
Households had a single income, and the thought of women not being housewives was shocking.
Then after a couple of wars where women started doing jobs which men did, couples realised that if they both worked, then they'd have more money, could afford a bigger house, more holidays etc, and still be able to rely on a single income while they had kids.
As more and more couples did this, house prices went up, the cost of living went up, unfortunately this meant that it became harder and harder to survive on a single income, so couples started putting off having kids, from their early 20s to mid 30s.
The population growth slowed, causing a social security timebomb for future OAPs.
Currently it is not easy to live a relativly comfortable life with just one income (if you're just starting out) so more and more women are having to go back to work earlier than their mothers did in order to make ends meet.
Now if you look at the cost of childcare, and the median earnings of women who need to use childcare in order to work, it can be as much as 50% of their salary.
Now, a mother at home looking after the kids isn't out working, isn't paying as much tax and NI, but a mother who is working and has kids in childcare (even with goverment vouchers) is paying tax and NI, and what's more, the childminder is now paying more tax and NI.
It may be a net tax loss (the voucher costing more than the tax paid by all parties) but it looks good on the employment figures and stimulates part of the economy slightly.
Sure you can argue that couples should wait until they can afford to have kids, and part of me agrees with that, but I also know that incog is right in that unless we start having more babies (in general, not just with incog) we're storing up problems for our old age.
The irony is, if women had stayed working in the home, then we probably wouldn't be in this mess