Some women simply don't want to be pregnant or to become mothers. There are countless reasons for this. Some are due to fears over health risks, real or imagined (and no pregnancy is completely risk free, let's face it). Some have had bad experiences with pregnancy and/or miscarriage already. Others are simply because they don't want to go through the physical experience, or don't see themselves as moms, or have other things they prefer to do. None of these reasons is wrong, but they can certainly be legitimate conflicts and a potential deal breaker with a partner who wants kids.
There are health issues that make pregnancy highly risky for some women, so it's discouraged, or (if it occurs) would have to be managed carefully.
Diabetes
Serious high blood pressure
Being very over or under weight
Abnormal pelvic structure (though of course c-sections help with this issue)
Having had many pregnancies already
A blood disorder like Von Willibrands, sickle cell anemia, thylassemia etc.
Hyperemesis gravidarium (past history of it means it is likely to be repeated in subsequent pregnancies)
Certain structural problems in uterus or cervix that make it hard to carry a pregnancy to term
Polycystic ovaries
Lupus or some other autoimmune diseases
Kidney disease
Most of these can be managed, at least somewhat, via modern medicine, but they do make pregnancy and delivery riskier and risk of miscarriage and dangerous complications for the mother more likely.
I'd hope that if the ob-gyny calls your character "missy" in the book your character then tries to strangle that ob-gyny with their own stethoscope.
Indeed. Though there was a time, not so very long ago, where this kind of patronizing behavior by doctors was more common. Maybe the novel takes place in such a time, or the doctor character is a relic.