Right, if your niece is a trainee at an American med school, I take it she's ... maybe 22? 23?
As in, besides not having children of her own, she may well not have any good friends who are female with children?
If that's her background today, then she may have a very, very limited perspective on the emotional roller coaster so many of us have about giving birth, and the shift in identity that brings.
Give her grace. She doesn't understand. She doesn't understand that you used to have this one version of life, the super-athletic basketball player on a team of women achieving things. She doesn't understand that version of your life has been upended because you've had children. She doesn't understand that you're having trouble seeing your old self in the mirror. She definitely won't understand any emotions you may be wrestling with.
It sounds like she may have taken your questions quite literally. With time and experience, she will probably figure out how to understand what a patient is really saying to her, and understand what they actually need. But not in her early 20s, head down and absorbing procedure.
Give her grace. That will lessen the impact of her comments.
Do you feel healthy? Do you feel in shape? Do you feel physically capable and strong? Focus on that.