The gender pay gap for people in their 20's has halved in a generation.
But according to a report by the Resolution Foundation it widens as women enter their 30's and 40's.
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See also: Gender pay gap halves but children still come with 'pay penalty'
Laura Gardiner, senior analyst at the Resolution Foundation says: "Research shows the progress through each generation on closing the gender pay gap for millenial women and those born since 1980, the gap has reduced to 5%.
"It's still there but just 5%, half what it was for the generation before.
"However, when these millenials turn 30, the gap opens up rapidly to nearly 10% which is exactly where it was for the generation before.
"Which suggests that the old challenges around childbirth persist for young women today."
Gardiner outlines there have been huge improvements in policy around maternity rights, child care and equal pay legislation.
She added: "Given that that gap remains, there's a lot more that we need to do.
"We need to think in particular about part-time working, why it's still overwhelmingly done by women and why there is so few opportunities for pay progression when people elect to work part time."