I'm not a doctor, but I don't think there's a "best" time to have kids in any profession. I think there are times that you will have more time and energy to raise a kid, but that doesn't necessarily mean it's the "best" time. In my opinion, the best time to have a kid is when you and your significant other discuss the options and come to a mutual decision.
However it would be difficult or impossible without a lot of help. Affording child care as a resident would be tough. Due to the necessity of taking overnight call a live-in nanny or similar setup would be necessary .
Waiting until you enter practice may be preferable since you can probably afford to pay for more help.
Jemima A.’s Answer
Thanks to assisted reproductive technology. You can have a child anytime you wish as long as eggs are cryopreserved (stored) for later use.
Although, this is not a totally guaranteed but success rate has been achieved by couples that practiced it.