From a philosophical perspective, your question is a difficult one. From a legal perspective, it’s easier. The actual holding of the Supreme Court’s decision in Roe v. Wade was as follows:
(a) For the stage prior to approximately the end of the first trimester, the abortion decision and its effectuation must be left to the medical judgment of the pregnant woman’s attending physician.
(b) For the stage subsequent to approximately the end of the first trimester, the State, in promoting its interest in the health of the mother, may, if it chooses, regulate the abortion procedure in ways that are reasonably related to maternal health.
© For the stage subsequent to viability, the State in promoting its interest in the potentiality of human life may, if it chooses, regulate, and even proscribe, abortion except where it is necessary, in appropriate medical judgment, for the preservation of the life or health of the mother.
According to CNN, Peterson’s wife was 8 months pregnant when she disappeared. Clearly, an 8-month old fetus is viable. Therefore, under the Supreme Court’s standards, the state is allowed to classify aborting an 8-month old fetus as murder.