Younger mothers suffer a 15% "pay penalty" compared to women who don't have children, a new study has shown.
Research by the TUC revealed that women who have children before the age of 33 earn less because they are likely to be out of work for a while, or return to a part-time job.
The report, published on International Women's Day, said younger mothers are also likely to face poor treatment at work, sometimes claiming to be forced out of their job because of issues around maternity leave.
In contrast, older women with children working full-time are more likely to earn more than childless women, often because they are in senior posts.
TUC general secretary Frances O'Grady said: "This research shows that millions of mothers still suffer the motherhood pay penalty.
"We need to do far more to support all working mums, starting by increasing the number of quality part-time jobs and making childcare much more affordable.
"Women in full-time, well-paid jobs shouldn't be the only ones able to both become parents and see their careers progress."
The University and College Union (UCU) said female staff in higher education were paid "significantly less" than men doing the same job.
At almost two thirds of English further education colleges, male lecturers were paid an average of £1,000 more than women, a study by the union found.
The pay gap was as high as 19%, with an average of 8% in the worst offending colleges, according to the research.
UCU general secretary, Sally Hunt, said: "These colleges and universities should not have allowed such shameful levels of pay inequality to persist. It's nearly 50 years since the Equal Pay Act came into force and they're still flying in the face of it.
"On International Women's Day, we would like to see a firm commitment from sector leaders to close the gap and are offering to work with institutions to make equal pay at every college and university a reality."
The Fawcett Society said a "motherhood penalty" still existed in UK industry, with views that mothers are less committed.
A survey of 8,000 people for the campaign group found that almost half believed women were less committed to jobs after having a baby, compared to 11% for men.
Chief executive Sam Smethers said: "It is clear that when a woman has a baby she is overwhelmingly perceived as becoming less committed to her job, while a dad is much more likely to be seen as more committed.
"This drives inequality and forces women and men into traditional male breadwinner, female carer roles."
A poll of 2,000 adults by Investors in People showed that eight out of 10 women believed gender discrimination is still present in the workplace.
Fewer than a third of men believed there was no gender pay gap despite official figures to the contrary.