Today is Equal Pay Day: the symbolic fiscal cut-off point in 2017. Thanks to the UK's 19 per cent gender pay gap, women effectively stop earning relative to men. In other words: you might as well go home.

That's right, your uterus is trying to attack your bank balance. And according to the Women and Equalities Select Committee, the number one cause of this gap is motherhood. But, before you drown yourself in your morning coffee, or scream something obscene at Bob from accounting, there is positive news.

Enter Joeli Brearley, the fearsome and funny 38-year-old founder of campaign group Pregnant, Then Screwed. This Halloween, she sent hundreds of working mothers marching on Westminster dressed as mummies (the scary kind), demanding equality in the workplace. The March of the Mummies drew widespread attention (even Keira Knightley asked to join) and was name checked at Prime Minister's Question time.

Pregnant, Then Screwed are fighting the myriad discriminations and injustices that befall working mothers: resulting in a terrifying average of 100,000 women citing pregnancy-based discrimination in the office and 54,000 mothers dropping out of the workforce annually.

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Brearley set up the group after she became one of them.

'When I found out that I was pregnant with my first child, I told my boss over email,' she tells me, 'The next day they sacked me by voicemail.' She shared her story with mothers she met at parent groups following the birth of her child. Far from being an isolated incident, she found that her story was one of many and was inspired to take action.

At its launch in 2015, Pregnant, Then Screwed was imagined as a virtual safe space for women to share these stories; many of whom fall silent, through vulnerability, isolation, or confidentiality agreements that amount to gagging orders. They now have over a thousand stories on the site, a legal advice service thanks to some 'very lovely' pro-bono lawyers, a flexible working advice hotline and a mentorship scheme, where women who have faced tribunals can advise those about to go through the process.

There are stories like Lisa Monroe, 26, who not only lost her job at a pub when she announced her pregnancy, but was literally barred from the establishment when she came back to contest it. Or Rita Lanning, 33, whose extra hours were so demonstrably incompatible with raising a child that she asked to go part-time, or on a job share. Without discussion of either she was fired.

'The majority of women I've spoken to have encountered some form of discrimination,' says Brearley, whose observation is strengthened by Pregnant, Then Screwed's rapid expansion from online forum to global agent for change. They now have outposts in the US, in Spain and even supposed parenting-nirvana Sweden.

The group's first foray into lobbying was their work on changing the restrictions placed on women seeking justice for employment discrimination, specifically elongating the legal three-month period in which you can raise a case.

Alarmingly, 14 per cent of women who said they had encountered discrimination were unable to challenge it because of this time limit, which typically falls during their pregnancy; a time of extreme vulnerability and potentially even illness. Pregnant, Then Screwed's petition on the matter raised 54,000 signatures. When brought to parliament it gained signatures from 103 MPs across all political parties. Though a promising start, it was still one of the causes the group marched for this Halloween.

Another demand made by the group is a call for subsidised childcare. Currently the government will subsidise from three years of age but, as Brearley points out: 'There's a gap between maternity leave of nine months and three years, with no subsidised childcare. What are you meant to do?'

Many mothers simply cannot afford to work, many can't afford not to work and, horribly, most are a mix of both. Added to this perfect storm is the fact that we have the most expensive childcare in the world which, according to a recent TUC report, has risen at four times the rate of the average salary since 2008.

Most pertinent of all, of course, is the fact that we see working mothers fall out of the workforce but not working fathers. The Women and Equalities Select Committee hit the nail on the head in its report last year, citing the major cause of the gender pay gap as 'women's disproportionate responsibility for unpaid caring.'

To combat this, Pregnant, Then Screwed have campaigned for flexible working by default and for both parents to have six weeks leave at 90 per cent of pay, which is presently only available to mothers. As Brearley astutely notes: 'What the government is saying by that is that childcare is more of a woman's responsibility.'

Men are stepping up, but workplace culture needs to follow suit. A third of fathers recently surveyed by the Fawcett Society said they felt unsupported at work and four in 10 lied to their bosses about taking time off to be with their kids.

'Fathers are still seen as the main providers for a family' says Brearley before telling me the grating statistic that men are actually more likely to get promoted after having children. 'There's loads of research which shows that, conversely, once a woman falls pregnant, she is considered less competent, less committed and deserving of lower salary.'

Susan Reid, 42, found this when her male maternity cover was promoted above her, as the new VP of marketing: 'I could return to my old job but the VP job now covered most of my job description and I would no longer be the marketing decision maker. At no point was I told that there was a potential promotion or offered the opportunity to apply.'

Reid then became just one more mother dropping from the workforce.

With a second large-scale event planned in April next year, Brearley's passion project has now become a force to be reckoned with and it seems only too right she should speak up on Equal Pay Day.

After all, as one of the best placards during their Halloween march read: 'The gender pay gap is a real mother f****r'.