The Labour MP revealed she has had two previous miscarriages and is now pregnant again expecting her first child.
Writing in the Guardian, she said the Independent Parliamentary Standards Authority (Ipsa), which regulates MPs’ pay, does not recognise that MPs go on maternity leave and does not provide for any paid cover for work carried out outside the parliamentary chamber.
“Humiliatingly, it is making me beg for extra staff funding - or give up any chance of spending time with my child to make sure my constituents don’t miss out,” she said.
“I never thought parliament would tell me to choose between being an MP and being a mum,” she said.
The MP for Walthamstow, in east London, said she continued working “aching and bleeding” during her first miscarriage and led a public meeting the day after her second.
“Now I’m pregnant once more and terrified - not just that it will go wrong again, but because I know that my resolve to keep my private and professional lives separate has become impossible,” she said.
“I’m coming forward publicly to talk about it because, as for far too many women, the personal inevitably becomes political when reproduction is involved.”
In January, a year-long trial of proxy voting for those with babies was approved after the issue was highlighted when Labour MP Tulip Siddiq postponed her Caesarean section so she could be wheeled into the chamber to vote.
Harriet Harman has previously called for MPs to get six months’ paid leave, in line with the Civil Service, and be able to nominate a full-time paid “maternity cover” representative for their work outside the Commons.
Ms Siddiq said: “As a politician, I’ve never stopped fighting for women to have control over their own bodies through the provision of reproductive rights and services as the non-negotiable prerequisite of equality.
“As a pregnant woman, this recent experience is another bitter reminder that it's still often men - this time the Ipsa executives - who will make the choices that determine if that battle will be won.”