Excuse me, but how did you think they calculated gestational age?

So there has been this thing, where, for some reason, the legislature here in my state has tried to pass several laws recently that have to do with procreation and abortion.

I will admit that I have a tendency to jump to conclusions about these sorts of laws and, partially because the topic isn’t a really big deal for me, I have a tendency to just listen to what the media is saying about a law. I generally assume that 50% of what they claim the law does is complete and utter bullshit, since that’s my default setting for news, but this time I actually started doing a little research because someone actually bothered to bring up a discrepancy in the facts when talking about the law and posted a link to the bill itself.*

The issue at hand seems to be this part of the bill.

On page eight of the proposed amendment to H.B. 2036, lawmakers lay out the “gestational age” of the child to be “calculated from the first day of the last menstrual period of the pregnant woman,” and from there, outlaws abortion “if the probable gestational age of [the] unborn child has been determined to be at least twenty weeks.”


(Don’t bother looking in the bill summary I linked to, the first “quote” doesn’t exist in the summary.)

Because of this, apparently, the state of Arizona is trying to be tricky and prevent abortion from week 18 not week 20, as the law suggests. Because two weeks makes it so much better *eye roll*

The only problem is, this isn’t some tricky language from legislators who are trying to sneak in a shorter amount of time for abortions to take place. This is how gestational age has always been calculated this way.

Gestational age, or the age of the baby, is calculated from the first day of the mother’s last menstrual period. Since the exact date of conception is almost never known, the first day of the last menstrual period is used to measure how old the baby is.

– American Pregnancy Association

Here’s what an OB/GYN had to say on why this is how the age of the fetus is calculated.

No, it doesn’t mean that the law prohibits abortion at 18 weeks. It means 20 weeks. Pregnancy has ALWAYS been calculated from the 1rst day of the last menstrual period. Always, always, always. When we do an ultrasound, we still use that convention. Perhaps it seems odd to non-obstetrical folks, however, since the last day of the period can be inaccurate (do you use flow? spotting? one spot of blood?), as can date of ejaculation (what if you have sex 3 days in a row?), and date of ovulation (how would you know that?), and the fact that fertilization takes 24-48 hours, the 1rst date of the last menstrual period is the only accurate date. Every other state law uses the last menstrual period (as does every obstetrical textbook) because that is the way we calculate gestational age.

Dr. Jen Gunter

So, technically, when things like this are said:

“Considering that it’s anti-choice nuts we’re talking about, it’s safe to assume that they’d simply prefer a situation where all women of reproductive age are considered to be pregnant, on the grounds that they could be two weeks from now,” RH Reality Check’s Amanda Marcotte adds in a recently-penned editorial. “Better safe than sorry, especially if that mentality means you get to exert maximum control over the bodies of women of reproductive age.”


The people that Amanda Marcotte should be going after are actually the doctor’s who staff the American Pregnancy Associate or the other medical organizations that have decided that gestational age is the most fool-proof way to calculate the age of the fetus.

This isn’t “anti-choice” nuts deciding that I might as well be considered pregnant now, just because I could be 2 weeks from now. This is sound medical reasoning.


Now, do I have problems with the law? Perhaps.

Mostly I just don’t see the point. Our current state law(to my knowledge) prohibits abortions after viability, except in case of danger to the mother’s life. I don’t have a problem with that, as I’ve written about before.

So while I don’t take issue with the actual time constraints as far as the 20/18 week “conspiracy” goes…can’t our legislature find something more important to worry about?


*I’m usually quite lazy, the best way to get me interested in something is to give me to relevant links first. Other wise I just forget about it and never bother to google to topic.



  1. The really stupid thing about this is that it’s dependent on the person who wants an abortion to give you a date further back than what the state will allow. I recommend to all women in Arizona wanting an abortion the answer to the question “when was you last period?” is “16 weeks ago” and dare the state to prove otherwise.

    • Very good point.

      I’m fairly certain the biggest issue at that moment isn’t whether or not you tell the truth. Though ultra-sounds can provide a certain amount of assurance on the age of the fetus, it wouldn’t be admissible as a reason to prevent abortion by this law. This is all on the mother and what she tells the doctor.

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