It does not matter if a fetus is a separate human or an organ of the mother's body. The mother's right to bodily autonomy grants her the right to offer, decline, or withdraw lifesaving tissue donation to any other person for any reason (or no reason at all). Consent is hers and hers alone to give and take.

If the fetus was instead her 10 year old child with leukemia, and she was providing blood and marrow donations to help fight it, she always has the right to choose to decline her support. She does not need to provide a justification; not wanting to do it is justification enough, even if that withdrawal results in the child's death.

Terminating a pregnancy is no different. The embryo/fetus is dependent on tissue donation via the umbilical cord to survive. If a woman wants to decline that life-support at any time, it is her right to do so.