The gender pay gap more than trebles when women reach their 30s, often because of a "motherhood penalty", research shows.
A report by the TUC found women of all ages earned less than men, but the difference was greatest after they passed 30, rising to more than 20% between the ages of 50 and 59.
Women were also twice as likely as men to be poor, the study showed.
TUC general secretary Brendan Barber said: "We all expect our wages to increase as our careers progress. But women's wages start to stagnate as early as their 30s and many are paying an unacceptable penalty simply for having children.
"Despite girls outperforming boys at school and at university, too many employers are still failing to make use of women's skills.
"This waste of talent isn't just hurting their take-home pay, it's harming the UK economy too.
"When women earn poverty-level wages, the whole family suffers.
"If the Government is serious about ending child poverty, it must raise family income by creating better paid, quality part-time work for Britain's 7.5 million part-time workers."
The report was published before the TUC Women's Conference, which opens in Eastbourne on Tuesday.
