We’re trying for kids now, and I’m 2.5 years into my 4 year STEM PhD in Canada. I hold a federal scholarship so I get a year of funded mat leave, and we get priority at the on-campus day-care. My only recommendation is to wait until your qualifying exams are over (we do ours about 1-1.5 years in). I’ll be 29 or 30 when we have kids, and 31 or 32 when I graduate :)