It may come as a surprise to many people that (among healthcare providers) length of pregnancy is not dated in terms of months, but rather in terms of weeks. Even more unusual to a casual observer is the fact that the first week of pregnancy is defined as beginning at the first day of the menstrual cycle and NOT the day on which the sperm was deposited in the vagina.
The convention for defining the length of a pregnancy starting from the beginning of the menstrual cycle makes sense for a number of reasons. The aspect that I would consider all reproductive right advocates to consider is this:
Dating pregnancy by the start of the menstrual cycle correctly reflects that pregnancy is, first and foremost, a process of a woman’s body and not something that is caused by sperm magic. The presence of sperm is necessary for pregnancy, but the uterine enviromnent has to be right for conception to happen, and that is a function of the woman’s menstrual cycle. The contents of the uterus at the time of conception will continue to be there for the remainder of the pregnancy and will continue to be important throughout the pregnancy process. Just because it isn’t part of the zygote/embryo/fetus, doesn’t meant it isn’t part of the pregnancy. The arrival of sperm is not the absolute ground zero of gestation.
When abortion rights are discussed in terms of weeks of gestational age, I am surprised to see reproductive rights advocates choosing to talk about pregnancy in terms of weeks since the arrival of sperm rather than the woman-centric (and medically standard) weeks since the start of the menstrual cycle. The fact that even many feminists think of pregnancy as the result of a man’s ejaculation rather than a result of any of the equally necessary processes of the woman’s body is a result of patriarchy being so pervasive in our cultural understanding of pregnancy that we will defend it just because no different thought has ever occurred to us. So… let’s stop that, shall we?