Women earn less than men at every stage of their career, rising to more than £8,000 a year by the time they reach their 50s, a new study reveals.
The TUC said the wage gap means women workers receive £85,000 less over the course of a decade, revealing the "pay penalty" they face.
The difference starts as soon as women start work, with an 18-year-old woman in a full time job earning an average of £1,395 a year less than her male peers, the research showed.
By the age of 30, the gap widens to over £3,000 and leaps to £7,200 for women in their 40s.
The TUC said the difference showed the impact of parenthood on women's and men's earnings.
General secretary Frances O'Grady said: "Women suffer a huge pay penalty over the course of their careers, which peaks in their 50s. At current rates of progress it will take decades for women to achieve pay parity with men.
"Having children has starkly different effects on men's and women's pay, with women earning less after having kids, and men earning more.
"Far more needs to be done to help mums get back into decent, well-paid jobs after they have kids, and to encourage dads to take on their share of caring responsibilities."
The TUC said progress on closing the full-time gender pay gap has slowed over the last few years, calculating that at its current rate it will take nearly half a century to achieve pay parity between women and men.
The union organisation said more should be done to tackle excessive working hours and support women when they become mothers.
A Government spokesman said: "The gender pay gap is the lowest on record and is virtually eliminated among full-time workers under 40 but we are committed to eliminating it completely. That's why we're taking action by requiring employers to publish their gender pay and bonus gaps for the first time ever from April next year."