I think people underestimate how long "making it" as a scientist takes. For most life plans, having kids while you're still in education or training is a crapp

I think people underestimate how long "making it" as a scientist takes. For most life plans, having kids while you're still in education or training is a crappy plan. For science, "education and training" can easily stretch to being a postdoc at age 40. Waiting for kids until you're 'settled' just isn't a justifiable demand for a career that takes 30 years to become established.