There's always more than can be said, but I get at some of the modern history in a previous response about the "pro-life" movement.

"Pro-life" is a fairly modern political movement that explicitly positioned itself in opposition to the abortion-rights or "pro-choice" movement in the 1960s and 1970s. Leaders leaned hard into appealing to bystanders' (ie. politicians', lobbyists', religious leaders' - not pregnant people's) emotions by making vivid appeals to a desire to protect pregnancy. In effect, the moniker "pro-life" was about shifting the focus from the pregnant person - who was Sanger's primary concern and the focus of the "pro-choice" movement - to the pregnancy, or the fetus. And more significantly, to the idea of an "innocent" fetus.

Sanger was interested in keeping pregnant people healthy and making sure those who could get pregnant had the knowledge and medical support to plan or space their pregnancies in such a way to prevent what she was witnessing on a daily basis, even in her own family. Sanger's mother was pregnant 18 times in 22 years. She gave birth to 11 living children and died at 49. And her experiences weren't uncommon. Fania Mindell, a white Russian Jewish immigrant who worked with Sanger, was responsible for writing and translating literature on birth control and interviewed dozens of immigrant women in the neighborhood around her clinic. She helped create flyers that spoke directly to the concerns raised by the women. One such flyer read, "Mothers! Can you afford to have a large family? Do you want any more children? If not, why do you have them? Do not kill, Do not take life, but Prevent. Safe, Harmless Information can be obtained of trained Nurses at 46 Amboy Street…All Mothers Welcome!"

The forceful and dramatic shift from the pregnant person to the pregnancy can primarily be traced back to a white, Catholic married couple, John and Barbara Willke. They created a book called "Handbook on Abortion" in 1972 (a year before the ruling in the Roe v. Wade case which prohibited states from making abortion illegal1) that was organized around images of aborted and miscarried fetuses. Their explicit goal, and the purpose of the images they collected, was to shift the public sentiment to view a prenatal fetus as indistinguishable from a postnatal baby.2 In other words, they worked to position themselves as fighting for the "life" in the pregnant person's womb, effectively minimizing the pregnant person and their health, life, and needs.

The 1973 "March for Life" was based on a similar rhetorical position. The founder's emphasis was on the idea of a fetus, not on the living pregnant person. In other words, the march wasn't about increased access to prenatal care for pregnant people, it was explicitly about drawing attention to the pregnancy and what they saw as the unnecessary taking of a life. This focus explicitly put those who identified as "pro-life" in conflict with Sanger's philosophy, feminists, and leaders of the pro-choice movement who focused on the pregnant person, including efforts to get them high-quality prenatal care. Another way to contextualize this is to compare statements from abortion access groups like the Jane Collective who put out advertisements saying simply, "Pregnant? Don't want to be? Call Jane" and statements from the first March for Life, "An estimated 20,000 committed prolife Americans rallied that day on behalf of our preborn brothers and sisters."