The pay gap generally kicks in after kids
Take a look at this paper: the long and short is that experienced workers are paid more, so if a woman takes herself out of the labour market to raise a child, she'll re-enter it at a disadvantage compared to her colleagues (male and female) who have continued working.

The Telegraph summarises the government's own statistical analysis of the pay gap, which points to it not really existing until women decide to have children.

There's a good round-up of other reasons for a perceived pay gap at economics blog Stumbling and Mumbling.

So women aren't, and shouldn't be, paid less because of gender and risk of pregnancy - but they do lose a competitive advantage if they leave work. The same would apply to a man who took a year off work to look after a child while the mother worked, or anyone who was off work for a long time because of illness: they'd lose ground on the competitors for jobs.
(Lapin, Mon 4 Jan 2010, 12:15, Reply)