In the march for trans rights, we have made a mistake by presenting a poorly worded idea: "biological sex" being solid, "gender expression" being the thing that changes.
When a transgender person's history is made known, their "biological sex" will be mentioned. The general idea is that one's biological sex is immutible and unchangeable, while their gender expression changes to align with their gender identity. But how does one scientifically and objectively measure biological sex? What makes someone "biologically male" or "biologically female"?
Before transgender identities were largely decided to be a political topic, rather than an issue individuals face, the general idea was that one's true sex would change upon getting sexual reassignment surgery. Some people will claim that, no matter what, somebody will always biologically be their birth sex. But there are always anomalies, and there are several ways of measuring what somebody is biologically.
I present to you a hypothetical: you meet a woman, born male, who has surgically changed her primary sex characteristics. Due to puberty blockers and having consistently taken estrogen, never even went through male puberty. She, externally, appears completely female. When her blood is tested, she has the hormone levels expected of a cissexual woman, and in fact when they checked her exact genetics it was discovered that she has XX male syndrome.
Aside from her lack of a uterus, which may even change in the future, there is no difference between this lady and an average lady. Would you call her biologically male, when there are very few physical similarities between her and a man you would on the street?
How many factors, in this case, need to be changed and measured to decide what is and isn't "biologically male" or "biologically female"?
The differences matter in different situations, of course. If somebody is taking a medication that will affect a woman's heart differently to a man's, it's not like the change is happening due to the presence of a vulva and uterus—it is likely hormonal, or due to body weight or composition. There are always tall and strong women of both types, though.
Would it be appropriate to warn a woman far into her transition of these possible effects? I think so. I think not warning her would be malpractice.
The point of being a transsexual, if not transgender, is to change one or more of one's sex characteristics. Therefore, the idea of "biological sex" not only being different to one's gender expression but being set in stone is entirely ridiculous.
If a transgender man takes hormones to induce male puberty, does weight training and adheres to diets to build a more masculine body with typically masculine fat distribution, has surgery to remove his breasts, uterus & replace his vulva with a phallus, he has undoubtedly changed his sex!