I can only give an example for developed countries. In the Ny Times article Why the Earnings Gender Gap in Business? Women Work Less from 2009 they cite research on MBA couples, and it's argued that wages remain largely the same among MBA holders up until the first child. After which most mothers (who continue their careers) are affected more than fathers when trying to balance motherhood and work.
Similarly in We did it! - The rich world’s quiet revolution: women are gradually taking over the workplace from the Economist also from 2009 argues that women who do well in their careers in their 20's, disappear in their 30's because they have to choose between motherhood and the career.
In the same issue of the Economist they give the statistics in the article Female power - Across the rich world more women are working than ever before. Coping with this change will be one of the great challenges of the coming decades:
Another American study, this time of women who left work to have children, found that all but 7% of them wanted to return to work. Only 74% managed to return, and just 40% returned to full-time jobs.
There are other differences between men and women in modern times that should create a shift in wages. In Sweden (my home country) 60% of university students are women according to the national university review of 2008 (in Swedish, sorry), and women typically do better than men in school {citation needed?}. I would argue that if university graduates earn more than non-graduates in the same job then womens wages will progress to the point where they overtake men if the above mentioned issue isn't a factor.
Some support that things have changed for women since 1970 was recently shown in Marginal revolution (a blog by two well published economists) where they plotted GDP in America together with the median wage by gender. They showed that while womens wages closely follow GDP, males wages have stagnated. If true this would mean that women are closing the income gap on males at the rate that the GDP changes.
More recently in 2015 a German newspaper reported on a German study that women work 23% fewer hours than men, with a wage gap of 22%. The work week difference of 7 hours doubles to a difference of 15 hours once there's children. It also says that of women with children 70% work part-time twice the amount of those without. Interestingly for men in Germany the change happens in the other direction where 6% of men with children work part-time, as opposed to 10% of those without. My German isn't good enough to read the original study for the researchers thoughts, but it seems to suggest that the wage gap in Germany is explained by the number of hours worked as opposed to wage discrimination.
I don't have a better answer than this, but I hope I've added a some food for others.
TL;DR
Women start earning less when they get children because they value parenting more.
If we are merely concerned with women as a whole demographic being underpaid then I would expect that they are going to rise above men as a whole demographic as we are graduating women in larger numbers and with better grades, which is a fairly recent happening.