The U.S. Supreme Court defines involuntary servitude as “the control of the labor and services of one man for the benefit of another, and the absence of a legal right to the disposal of his own person, property and services” (Plessy v. Ferguson, 163 U.S. 537, 542 (1896)). Again, “a condition of enforced compulsory service of one to another” (Hodges v. United States, 203 U.S. 1, 16 (1906), and also “that control by which the personal service of one man is disposed of or coerced for another’s benefit which is the essence of involuntary servitude” (Bailey v. Alabama, 219 U.S. 219, 241 (1911). The doctrine has been endorsed by the U.S. Court of Appeals, even more clearly: “The essence of slavery or involuntary servitude is that the worker must labor against his will for the benefit of another” (Wicks v. Southern Pac. Co., 231 F.2d 130.138 (9th Cir.) (1956).
If the U.S. Constitution could ever be construed as implying the fetus to be a person, anti-abortion laws would end up in a person—the pregnant woman—being compelled to perform services to another—the fetus—against her will. That’s involuntary servitude.
Every American citizen is protected against involuntary servitude by the Thirteenth Amendment. Now, your theory is that the woman does alienate such protection by performing sexual intercourse. No, she does not. Constitutional rights are unalienable.