Danger to the doner is irrelevant. Forcing a parent to donate blood to its child or stranger is still impossible, so is forcing someone to give bone marrow and those aren't very risky procedures.
Danger to the donor is never irrelevant. Ethical concerns aside, it also raises an inescapable constitutional issue of deprivation of life or liberty without due process.
Remember, bone marrow transplants are a surgical procedure. Even though they are low-risk compared to other surgeries, all surgeries carry some risk of death or serious injury.
So what? From organdonor.gov: There are lots of death related to the need of organs. Why not force living people to give organs, blood or bone marrow if it doesn't kill them? Why not "force" the dead who really do not need those? Why not at least force parents when it is there own kid? Why stop "forcing" at birth?
Again, in the USA, there is no affirmative duty to save the life of another unless it is your job or you placed the person in jeopardy of loss of life in the first place.
If a kid needs a kidney and is dad is a compatible donor, the kid still has no say in what his dad will do and he has a better legal status than a zygote.
Is the dad being an ass? Sure, but that ain't illegal.
In most- not all- cases, if the Dad says no, the kid still has a reasonable chance of survival if another donor can be found. A familial donator is always preferred, but isn't necessary.
In contrast, with current medical technology, a child in utero essentially has no other option for survival outside the womb until it reaches a certain age. Even then, premies are at increased odds of certain medical conditions the rest of their lives.
There are all sorts of risks associated with pregnancy, some less apparent than others. Necessity is relative. But that point is irrelevant.
I'm well aware of the risks. And risk is never irrelevant.
It is a woman's uterus, she does what she wants with it.
In the USA, no right is absolute. The is always a balancing act between rights and duties, between individual needs and those of the population in general/the state.
And yes, I do fully realize that in this case, we are talking about the state invasively protecting a person/something* within the body of another human, so the bar has to be set pretty high. My personal, considered opinion is that when the taking of life is involved, the stakes are legitimately high enough to warrant it, at least granting a stay of execution until the baby is viable and can be placed for adoption.
* depending on your view of the personhood of a fetus