Women who choose to have children aged 33 or younger are sacrificing part of their pay packet, a study has found.
Dubbed the "motherhood pay penalty", the Trade Union Congress found these women will earn 15% less than their peers who haven't had children.
However, women who have their first child at 33 or older have a wage increase of 12% compared with women the same age who haven't had children.
The TUC analysis, which was carried out by the Institute for Public Policy Research to mark International Women’s Day, concluded that the "pay penalty" is caused by younger mothers being "more likely" to have a significant period out of work.
"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," Frances O’Grady, TUC's general secretary said.
"Women in full-time, well-paid jobs shouldn’t be the only ones able to both become parents and see their careers progress."
Researchers used statistics from the 1970 Birth Cohort study and compared mothers' full-time earnings with those of women with similar levels of education and in similar full-time jobs but without children.
The research is forming part of a wider TUC report on working parents to be published later this year.
To address the pay gap for mothers, TUC wants more free childcare to be available from the end of maternity leave and greater support for more equal parenting roles.
The study report stated: "The introduction of shared parental leave was a step towards this but because of the poor pay and restrictive eligibility requirements for fathers it is likely to have limited impact.
"We need some better paid, fathers’ only [rather than shared] leave."
This isn't the first time the idea of a "motherhood penalty" has come to light.
In December 2015, a study found mothers who return to work after maternity leave get less pay and fewer promotions for "decades".
According to research from the Chartered Management Institute (CMI), mothers are cornered into "softer roles". The CMI also dubbed this the "motherhood penalty".
Rosalind Bragg, director of Maternity Action, told The Huffington Post UK at the time: "Finding a new job when you are pregnant or caring for a young child is challenging.
"Women often face overt discrimination from employers and many are unable to find jobs at the same level of pay and responsibility.
"We should focus on preventing unfair and unlawful treatment of pregnant women and new mothers at work, rather than relying on individual women to pursue expensive legal action against their employer."
A UK government spokesperson said, according to The Guardian: "We’ve gone further than ever before in tackling the gender pay gap – and the gap has been virtually eliminated for women under 40 working full time.
"We are also introducing a range of measures to help both mothers and fathers balance having a family with their career, such as our plans to offer 30 hours of free childcare for working parents, the extension of shared parental leave and the right to request flexible working."