They're the women who know exactly what you're feeling, offer you advice just when you need it, make you laugh when all you want to do is cry.
So whether you call them 'mummy friends' or not, here are 10 pretty good reasons on why it's so great to have those women in your lives from our HuffPost UK Parents mum bloggers.
1. You can talk about your kids as much as you like.
You know that friend of yours with no children? Yeah, she's probably getting bored of you telling her how your daughter is finally eating solids.
Blogging on HuffPost UK Parents, Claire Kirby wrote: "They don't get bored and start rolling their eyes and stifling yawns when you talk about your kids. You can even tell them about poo and vom and they won't get grossed out.
"You can ask them random questions like 'is it normal that my small person can't dance without smacking his own butt repeatedly and changing random lyrics to 'poo poo head'?' They are like Google. Only without all the gross images."
2. You have people to go out with.
"You look around at your four mum mates. It was meant to be eight but three dropped out due to sick children, no babysitter, relationship issues or got pregnant again and so on," wrote Emma Conway on her blog about going out as a mum on HuffPost UK Parents.
"Yet you are rabid that five actual human beings made it out. Together. On the same night. AND ONE OF THEM IS CHILDFREE AND WILL HAVE ACTUAL STORIES TO TELL.
"You treat yourself to another rosé and take a selfie to put on Facebook to ensure everyone knows you still have a life."
3. You can be honest.
Really honest. I mean, does your childless friend want to know about what happens to your vagina when you've given birth?
"You spend approximately 15 seconds getting to know one another. You've talked about your vagina before you've mentioned what you do for a living.
"And there's absolutely no one I'd rather talk about my vag with. My rubbery, old nipples. My crazy obsession with Gina Ford."
4. There is no judging.
"It's almost six years to the day that we met. We now have 19 children between us," wrote Amy Ransom.
"And despite house moves and life getting in the way, not once have we not been there for each other. Not once have we judged each other. Not once."
5. They'll be there when you need them.
"As a woman who never understood why Mums had to have 'Mum friends'. I now do," wrote Ceri Gillet.
"Embrace all women and let them gather around you. Take from them their wisdom, let them share in your experiences and hold a space of quiet understanding, no judgment. Form your sisterhood.
"They'll always be there."
6. They can cure spouts of loneliness.
"Parenting can be a lonely job sometimes, and we all need friends around us," wrote Clare Mackintosh.
"In the world of parental dating there's no need to worry about two-timing, you can forget the make-up, and no one cares if you've shaved your legs. Go on: make a date with a mum today – you'll be glad you did."
7. They help you have a break.
Why? Because those playdates you go on mean your kids play together and you can focus on yourself. Just for a bit.
Writing about a playdate, Lisa Sadikman said: "We let the girls run around, we glance at them and lob warnings over the empty tables, admonishing them as expected, but really, neither of us care that much.
"They aren't hurting anyone (yet). They aren't tripping up any of the waitstaff (yet). They aren't that annoying (yet). And we need a break.
"We need those moments between us to shore ourselves up, pool our lives together for a few minutes, swim in the shared love and frustration of motherhood and know we are holding life jackets for each other."
8. The advice.
You meet those mums that know the answers, they have the answers. They've either done it before or they've experienced it recently. Either way - you can go to them for advice.
Blogging on HuffPost UK Parents, Debbi Marco wrote about the mum who's "already done it" who will be there to give you what you need to know.
She wrote: "Whether it's crawling, eating or sleeping through the night, you can be sure as soon as you mention it, her baby has already achieved it.
"If her baby really is the leader of the pack, you can get some good tips and learn from her mistakes."
9. They're there for the mini breakdowns.
"When it's been a day full of tears and tantrums, (and not just the small people) mummy friends understand, and they always have cake. The really good ones always have wine," wrote Claire Kirby.
10. They start as 'mummy friends' but some end up as true friends.
"You just click and bond over all things mummy-related," wrote Laura Powell-Corbett when describing the 'mummy soulmate'.
"Before you know it they become a true friend, one you laugh with, cry with, drink with, eat too much cake with, wipe baby poo off all conceivable surfaces with.
"Once you find this one make sure not to let them go!"