Theresa May is being urged to live up to her "champion of women" image by ditching plans to limit tax credits to a family's first two children.
SNP MP Alison Thewliss wants the Prime Minister and new Work and Pensions Secretary Damian Green to "reflect" on the "anti-family" policy, which opponents fear will require rape victims to have to justify themselves to the taxman to secure child tax credits.
There are also concerns that different religious groups will be unfairly affected by the plans, due to be implemented from April 2017.
Then chancellor George Osborne announced the idea as part of the summer Budget 2015 in an attempt to slash the tax credit bill for new claimants.
The Budget document noted the Department for Work and Pensions and HMRC will develop "protections" for women who "have a third child as a result of rape or other exceptional circumstances".
But Ms Thewliss has voiced concerns about the lack of detail on the protections, as she prepares to lead a parliamentary debate about the reforms on Wednesday.
She told the Press Association: "I would like it if they scrapped the whole thing but I'm not sure if they will do that.
"Theresa May has rolled over on some things David Cameron was keen on. She's looked at things again, including on Hinkley Point C where she took a wee bit of time before taking a decision.
"I hope by raising this again, laying all the background out, the Government can see it's a terrible policy, it will not work, let's change it."
Ms Thewliss added: "Theresa May styles herself as a feminist and a champion of women.
"This is a policy that hits women hardest. I would hope, at the very least, they would give the opportunity for a bit of reflection and look at this policy again."