Ladies -- (speaking directly to the women here who want to have children and are contemplating when)
I am, as you may have guessed from my avatar and screen name, rather older than most of you, and have seen a thing or two along my journey. And knowing what I now know, if there were only one piece of advice I could give to my teen daughter, it would be this: Secure your own financial future before having a child. Specifically, do what you need to do to safeguard your own individual earning potential so you will have the power to make your own personal life decisions independent of your hypothetical future spouse. Don't ever forfeit your choice.
So ladies, I would absolutely advise not having children before medical school unless you somehow have the financial resources (prior earnings, parents?) and support system to complete your education and also pay for needed childcare in the event of a divorce. I'd even argue against 4th year or PG1 because you would still face great hardships to obtaining full licensure as a single parent. After you're fully credentialed and can support yourself as a physician -- then you have options.
Divorce happens, and sometimes it's for the best and sometimes it's an unavoidable disaster. But it happens, and the last thing you want to do is have to choose between staying in a bad marriage to support your children and abandoning your education and career prospects to pay the bills now because your responsibilities as a parent force your hand.
Having children geometrically increases the number and variety of potential complications in your life, and most of those problems can be solved with an infusion of money. Wait until you have the money to have the baby.