The Arizona law restricting abortion that is currently being held up by a U.S. Appeals Court is frequently described as redefining the start of pregnancy. It’s mocked for saying pregnancy starts with starts with the first day of a woman’s last period before she becomes pregnant. “What kind of idiot would declare that pregnancy starts then?”, people ask.
Well, the answer is, “Doctors”. I’ve been meaning to write a post explaining this for a little while, but geengeek beat me to it:
In the first case-based class of medical school, students are asked to answer a virtual patient’s question about the development of the fetus. These students are smart and they know all about betaHcG and are anxious to showcase their knowledge of the menstrual cycle with fluctuating levels of various hormones (FSH, progesterone, etc.). Yet one question brings confusion, “How pregnant is this women?” The related question, ”When does pregnancy start?” leaves the students flummoxed. Is it at conception? But how do you know when that happens? Or does implantation make more sense? It’s a great example of how detailed facts need the larger context.
The usual dating is gestational age, based on the first day of your last menstrual period. However, you can also date a pregnancy with embryological age, starting at conception.
How you date a pregnancy can depend on your perspective. My very general guideline:
- Pregnant woman is the focus = gestational age (e.g. obstetricians) 1
- Focus on embryological/fetal development = embryological age (e.g. developmental biologist) 2
But why are there two types of dates? We might need a bit of a primer on the menstrual cycle and how it relates to pregnancy.
The rest of the post is interesting as well, and far better than I would have done as a lay writer. Check out the whole thing.
There are definitely problems with the Arizona law. It places restrictions on when a woman can opt to have an abortion that don’t consider the timing of important tests and don’t match any remotely reasonable boundary in pregnancy, such as viability, that might be cause for debate. They instead plant a flag to declare the state’s interest in a pregnancy at some early, arbitrary point.
That’s quite bad enough. We don’t have to accuse them as well of an underhandedness they didn’t demonstrate.