A detailed study of 2,000 women who have put their careers on hold found that six in ten earn less now than they did before their baby arrived.
Nearly three-quarters are now in jobs they are over-qualified for and which they would have considered 'beneath them' before falling pregnant.
A typical working mum now earns £9,419 per year less than before giving birth and some of those polled were £20,000 out of pocket
The research, which was commissioned by AAT (Association of Accounting Technicians), found nearly a quarter of mums are forced by necessity to take any job that can help make ends meet.
Yesterday AAT Chief Executive Jane Scott Paul said: ''Motherhood is one of the biggest and best things a woman could wish to experience but there tends to be a perception that you can't become a mum and maintain the career you'd like.
''Clearly there are big financial implications for women who are forced to leave their previous careers and instead take up jobs that allow greater flexibility but that aren't challenging them professionally.
''Often this means a substantial decrease in the amount they are able to earn which is clearly having an impact both on their working happiness and the family finances.''
A third said the difference in salary has affected their life negatively and one in seven said it has repercussions on their marriage.
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Four in ten working mums do their current job because it fits around family and helps bring in extra money. Just 16 per cent of working mums were actually passionate about their current job.
Six in ten mums now find themselves in a completely different role to the one they did before falling pregnant and 75 per cent feel they have more potential than their current job allows.
And while working hours were obviously reduced in order to balance parent responsibilities, a fifth of mums said their current job was much less stressful than their previous positions.
One in three mums said the job they do now is a little menial and very different from the career they had before.
The average respondent had their first child at the age of 25 and, at the time of falling pregnant, 38 per cent thought the company they were with wasn't supportive enough throughout the pregnancy.
And one in five feels they would change the age at which they fell pregnant if they could - the most common regret was wishing they'd got a little higher up the career ladder before having children.
A third of mums feel they actively had to sacrifice their career in order to have children with a fifth convinced managing a family and pursuing a proper career simply wasn't feasible.
Indeed, four in ten find combining being a mum and a professional as a big challenge day to day.
Just a quarter aims to one day reach the level of income they were on before becoming a mum, with more than half admitting their priorities had changed.
Flexibility around school hours, job location and low stress levels were found to be the biggest priorities for working mums, with the salary it pays just fourth on the list.
Jane Scott Paul added: ''The results show ambition is still there but a shift in priorities means they need more flexible working roles in order to be able to manage a family while still developing their career.
''The good news is that many women take this as an opportunity to retrain and learn new skills - making them more employable across a range of sectors.
''Ideally, women want to be challenged and passionate about their job, while still earning a reasonable income and enjoying the everyday perks of being a mum.''