Babies. The reproductive division of labor is real. Whether it is good or bad can be debated by childless students in sociology classes across the globe but, in the end, parents make the decision that seems right for them. This often puts the childcare with the women and income generation with the men. This is more efficient than an even split.
Many couples decide to have children in the late twenties, early 30's. This is prime time for specialization in medicine. Hard to get ahead when you're balancing childcare with your career, it's basically like having 2 careers. I'm not saying it's impossible, but it's a pressure.
Of course, as things get less economically scarce, there are things like daycare, nannies, etc. that help with childcare and make it easier for women to work.
Even women that want nothing to do with childcare often have to leave work for 12 months or so, some take even a longer break.
Multiply it by 2-3 times and on average a woman has 3-4 years less than a man in her resume at 35-40 it can amount to 25-30% of your career since at that age it's likely not to be much longer than 15 years at that point.
Many couples decide to have children in the late twenties, early 30's. This is prime time for specialization in medicine. Hard to get ahead when you're balancing childcare with your career, it's basically like having 2 careers. I'm not saying it's impossible, but it's a pressure.
Of course, as things get less economically scarce, there are things like daycare, nannies, etc. that help with childcare and make it easier for women to work.