You are making a false analogy.... the natural process by which the female body prepares itself for pregnancy, known as menstruation, is a fundamentally NATURAL process, whereas an abortion performed by another person us fundamentally UNNATURAL.
There are pregnancies that end involuntarily without interruption, that are not carried to term with the delivery of a living baby. This is at times a natural process. What is NOT natural is having a "doctor" essentially reach inside that safe haven of the womb and pull the baby out artificially.
There is no "choice" involved in the natural functioning of the body, God made it to function as it does. What pro-life people like me abhor is when that natural process is unnaturally interrupted.