Laura Davies spent six years working for the same retailer without being promoted despite her responsibilities increasing significantly in this time.
When Ms Davies asked her manager why she hadn't progressed, she was told "I wasn't really the management type".
The 27-year-old mother, who works part time, is one of many women whose caring responsibilities have held them back.
One in four firms consider if a woman has young children or is pregnant in promotion decisions, a study suggests.
The research from charity Young Women's Trust - based on interviews with 800 managers by polling firm YouGov - found that a significant number were hesitant about hiring women in their 20s and 30s who might have children in the future.
The charity, which works to support women aged 16-30 on low pay, is urging firms to recognise that making hiring or promotion decisions based on such factors is illegal.
Ms Davies, who has a seven-year-old child with her partner, eventually left her retail role to do freelance online marketing.
"I came to the realisation that because I was part time, that I would never be considered for promotion. I realised it wasn't the right environment for me," she says.
She said the fact that she was a mum meant that her manager simply assumed "she never wanted to progress in her career".
Ms Davies said in the time she worked for the firm, others who started after her progressed much faster, being given "more training and more responsibilities".
"Because they were single and had no dependents they dedicated their whole selves to the job. Because I was part time and had set hours they almost forgot about me," she says.
Young Women's Trust chief executive Dr Carole Easton said employers with "outdated, discriminatory views" were missing out on important talent.
"Employers should value young women's contributions to their workplaces and do more to accommodate them, including by offering more flexible and part-time working opportunities," she said.
Ms Davies said working for herself had been "a breath of fresh air".
"It's sad I got to the point where I had to believe in myself because no one else would," she says.