If bodily autonomy is defined as the principle that no one is allowed to use another person’s body to survive, conjoined twins expose a serious limit to that claim. In cases where two people share bodily systems, one twin’s autonomy does not include the right to lethally control or kill the other, even if one is stronger or more dependent. The shared-body reality constrains autonomy because there are two equal human beings involved, not one.
Pregnancy can be understood as a similar kind of shared biological existence. It involves two people whose lives are intertwined through a temporary, shared system rather than a one-sided act of taking. Both the mother and the fetus actively contribute cells to form the placenta, which functions as a shared organ during pregnancy. This organ regulates critical aspects of health for both parties: it transfers nutrients and oxygen to the fetus while also producing hormones that support the woman’s body, help regulate metabolism, and increase energy reserves.
In both cases—conjoined twins and pregnancy—the moral intuition is the same. A relationship of physical dependence does not grant one person the authority to end the life of the other. If bodily autonomy does not justify killing a dependent conjoined twin, then it likewise cannot justify killing the fetus in pregnancy. Shared bodily dependence limits autonomy, but it does not erase the equal right to life of the weaker or more dependent person.
Key Takeaways
Bodily autonomy has moral limits in shared-body situations and does not include the right to kill another innocent human being.
Conjoined twins show that dependence does not cancel personhood or justify lethal control by the stronger party.
Pregnancy is not a one-sided use of the mother’s body but a cooperative biological relationship involving a shared organ, the placenta.
If killing a dependent conjoined twin is impermissible, consistency requires rejecting abortion as a solution to bodily dependence.