For me, childhood issues that I hadn't resolved resurfaced when I became a mother. I returned to therapy about 5 months after I had my daughter (and have been going ever since). Some of that was due to intense postpartum anxiety (and postpartum depression, though I didn't fully recognize that part at the time).
My daughter's initial diagnosis of a developmental (speech mainly) delay, followed by her diagnosis of autism dredged up even more stuff, like my dealing with OCD and anxiety as a kid, but not having help for it. Well, and things with ex-MC triggered some of that, too, in his talking about helping his daughter with her anxiety.
I was talking to Dr. T recently about why I became very attached to a teacher in high school, then didn't really have any sort of paternal/maternal attachment again until ex-MC. I thought at first that maybe it's because I was becoming attached to people I dated instead. I think that was part of it. But then I realized that a big factor was becoming a parent. He agreed that it made sense. I do wish I'd worked on some of this stuff more before becoming a mom.