Theresa May left MPs in stitches after mistakenly congratulating Jeremy Corbyn on the birth of a granddaughter.
The Prime Minister blamed a Tory ally for giving her the wrong information which left her laughing at the despatch box.
Labour leader Mr Corbyn had praised party colleague Conor McGinn (St Helens North) for delivering his own daughter on the living room floor after his wife gave birth more quickly than expected.
Mr McGinn shared his story over the weekend, explaining he was given a "crash course in midwifery over the phone with a 999 call-handler" in order to deliver Neasa Constance.
But Mrs May appeared to misunderstand the congratulations extended by Mr Corbyn, resulting in the mix-up.
Speaking during Prime Minister's Questions, Mr Corbyn said: "Could I take this opportunity in welcoming Neasa Constance McGinn and hope that the evidently effective crash course in midwifery undertaken by my honourable friend, the member for St Helens North, isn't a sign to the Government that we believe in downgrading midwifery training."
In her reply, Mrs May told Mr Corbyn: "First of all can I congratulate you on the birth, I understand, of your granddaughter.
Mrs May then started laughing along with other MPs as it dawned upon her that she had made an error.
She added: "In that case can I just say that perhaps one should never trust the former chief whip."
Mrs May then turned to Conservative MP Sir Patrick McLoughlin, who held the role for the Tories while in opposition and is now party chairman.
Mr Corbyn later replied: "It's a bit unfair to blame a former chief whip for a little bit of confusion - very ungallant.
"Can't we just admire the member for St Helens North on his work?"
Mrs May referenced the sacking of Dame Rosie Winterton as Labour's chief whip, as she countered: "At least my former chief whip has got a job."
Dame Rosie lost her position during Mr Corbyn's latest shadow cabinet reshuffle although she has since been made the party's envoy with responsibility for relations with Labour's international sister parties.