Having children has a direct impact on women's salaries, resulting in them being paid up to 20% less than their male counterparts, the TUC says.
The gender pay gap increases after women have children, says the TUC
A report, published on the eve of the TUC's Women's Conference in Eastbourne, says the gender pay gap more than trebles after women pass 30.
TUC General Secretary Brendan Barber says women are "paying an unacceptable penalty simply for having children".
The government is promising measures later this year to reduce the pay gap.
Mr 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.
"Despite girls outperforming boys at school and at university, too many employers are still failing to make use of women's skills."
The TUC study found that women of all ages earn less than men and that woman are twice as likely as men to be poor.
The wage gap is greatest between the ages of 50 and 59.
The Fawcett Society, which campaigns for equality between women and men in the UK, called the study's findings a national scandal.
Fawcett's campaigns officer Kat Banyard said: "At every level in UK workplaces women are being paid less than men.
"The paucity of senior flexible roles and the long working hours culture shuts women out of the boardroom and forces then into lower-paid, lower-status jobs when they have children."
Minister for women Harriet Harman said the government planned "tough new measures" in an Equality Bill to be published later this year.
"I just don't believe women are less committed, less hard-working or less able than men," she said. "So they shouldn't be paid less."