The pay gap that most people care about is that women earn less than men who do the same job. Because men tend to work different jobs than women, this statistic is not particularly interesting to me.
http://www.govtrack.us/congress/bill.xpd?bill=h110-1338
(For those unfamiliar, this law would have encoded the notion of "comparable worth" into law. Comparable worth is the notion that entire fields/job categories are underpaid due to sexism.)
"Once you control for factors like education and experience, notes Francine Blau — who, along with fellow Cornell economist Lawrence Kahn, published a study on the 1998 wage gap — women's earnings rise to 81% of men's" - http://www.time.com/time/nation/article/0,8599,1983185,00.ht...
"You can control for all you want to control for, and the gaps remain," - http://www.businessnewsdaily.com/women-intensely-dissatisfie...
There are statistics that give pay per hour, too.
The argument that the data supported is as follows: fewer hours worked (counting the expected or not unpaid overtime) led to smaller raises and fewer promotions. Subsequent raises and promotions are based on the resultant salaries and titles, leading to a snowball effect. Maternal leave puts the whole process on pause and puts a mother behind on the snowball process.
think about it for high paying professions such as top law firm work, or an investment bank -- getting ahead to make partner means working tons of hours which is not a viable option if you have a kid (unless there is a stay at home father).
A favorite trick though from the radical feminists is to lump all the men into one group and all the women into another group and then compare the two. Always, the men will come out on top simply because of biology. Women stay at home more than men starting with pregnancy, then staying at home with the toddler, a lot of women also choose to work less than 100% in order to be at home with their children, etc.