Research based on in-depth interviews with 40 people has found that having children is the single most significant factor in altering their friendships – for better and for worse.

For some who took part in the study, having children had proved catastrophic for some of their existing friendships.

As one put it: “Then she had children and our friendship changed dramatically…she became very child focused and that was quite difficult because she’d talk about her children all the time and I didn't... In astronomical terms, she’s like a dying star of a friendship.”

But, according to the study, motherhood also ushered in a new type of ‘domestic’ friendship, based almost entirely on the shared experience of parenting, often involving shared childcare and deep, albeit sometimes temporary, connections.

Another interviewee said: “Things have changed in our relationship because we were very dependent on each other when the children were little cos we used to cover for each other for pick up from school or drop off at school. We had a very, sort of, symbiotic relationship but as the kids have got older that’s been less important, I suppose. We’re still friends but probably less so.”

Sociologist Dr Anne Cronin, Lancaster University, found the type of friendships described by ‘mum friends’ had been less visible to sociological analysis than other types of friendship because they don’t fit the mould.

She said because ‘mum’ friendships were based on connections made through children these relationships changed dramatically over time and could be lost when circumstances changed. But such friendships also challenged the idea of the ‘exclusive’ intimacy of the couple or family unit; instead ‘mum friends’ often expanded the domestic unit, becoming part of it through shared childcare and mutual exchange of support.

She said: “The phrase ‘you can’t choose your family, but you can choose your friends’ is something we often hear. But mum friends are not exactly chosen – unless it’s by the kids’ own choice of friends – and yet they can become friends for life.

“Mum friendships can involve sharing everything – childcare, emotional upheavals, worries and pleasures – maybe even more than you’d share with family or your partner.”