I was talking to my SIL about it. She has recently gone back to work after her third child, and has been shunted into a sideways role.
She said it was distinct after her first and second child, but is more apparent now, and there is a difference in the career trajectory in her childless colleagues and those who have had children.
Just thinking about where I have worked, a lot of them on the surface have made a great song and dance about equaliyt and maternity leave provisiobs etc, however a lot of the women with young children have been given crap commodities/portfolios/clients, sidelined into a non-essential role, give easy stuff to do. These are both FT and PT workers. Of course some of these women may well have asked for a less stressful role, but I know that some of them were quietly disgruntled. I don't know if they managed to get off the 'mummy track' and claw back their careers.
I know if I had a baby now it would screw up my career, in spite of the fact I have a very friendly workplace, full paid maternity leave for 6 months, flexible working etc, all that jazz. But all the soft stuff you can't define - it would change, and my career prospects would be affected.
I don't know if I am overthinking this or not, but I wonder how many workplaces are like this.