Anneleise Dodds, shadow women and equalities secretary, proposed the idea at a Labour conference fringe event organised by motherhood campaign Pregnant Then Screwed (PTS).
She said the time limit extension was necessary in particular to give pregnant women who may have been unfairly dismissed more time to get redress.
Research found one in nine women said they had been fired or made redundant when they returned to work after having a child, or were treated so badly they felt forced out of their job, according to the Department for Business, Energy and Industrial Strategy (BEIS).
However, fewer than 1% of women who experience maternity discrimination raise a tribunal claim.
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Joeli Brearley, founder of PTS, said mothers who are mid-term are often forced to decide about bringing a claim while still pregnant.
Brearly told HR magazine: “For many women discrimination begins during pregnancy, which was the case for me when I was sacked the day after I announced my pregnancy. I then discovered I had a very high risk pregnancy. I had to reduce stress or risk early onset labour.
“Not being stressed was not a choice I had the privilege of making due to being pushed out of my job, and the three month time limit left me with the stark choice of protecting the health of my unborn child or accessing the justice I so clearly deserved.
“I dropped the case, of course I did. It wasn't really a choice at all.”
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The reform would help both employers and employees, according to Thomas Beale, a partner specialising in bullying and harassment claims at law firm Bolt Burdon Kemp.
He said: "Increasing the time limit for bringing claims removes the pressure to rush into litigation, and allows more time to process the potentially traumatic experiences they have been through.
“It would also give time for a more cooperative and constructive dialogue between the employer and employee, making it easier to settle claims outside of the tribunal system.”
