When Pregnancy Starts

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.

When Pregnancy Starts
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11 thoughts on “When Pregnancy Starts

  1. 1

    slight tangent: also from the AMA News link above:

    In South Dakota, physicians are required to tell women seeking abortions that the procedure causes an increased risk of suicide, according to a new law. The 8th U.S. Circuit Court of Appeals in July upheld the provision, saying that informing women of suicide risks is neither an undue burden on abortion rights nor a violation of physicians’ free speech.


  2. 2

    To calculate the age of the pregancy starting with the first day of the last period is a trick, since that’s usually the only reliable date you have (unless it was an IVF).
    It’s like bankers calculating interests on the basis of a 360 days year.
    But that doesn’t mean that anybody who knows anything about this claims that a pregnancy actually starts then anymore than any banker claims that that’s the actual number of days in a year.

  3. 3

    I really think they should have used menarche as the dividing line, since the whole point of the law is controlling sexually-active women. A little early for most, you might say, but better to err on the side of over-restriction when you’re dealing with wimmin.

  4. 4

    Giliell is correct.

    The problem with embryological age (counting from conception) is that unless you conceived through IVF it’s difficult to know with any accuracy exactly when conception happened. Counting from the first day of the mother’s last menstrual period is easier, since most women have that information and it tends to be within a few days of two weeks before conception. So rather than counting up to 38 weeks from conception, obstetricians just count up to 40 weeks from the mother’s LMP. Setting a date is important for legal and decision-making reasons later in the pregnancy, and this is the most reasonable way to do it unless the mother is sure exactly when conception happened and the difference between the two methods is more than a few days apart.

    It doesn’t mean that any doctors actually think that life begins at the menstrual period, though.

  5. 5

    Thanks for pointing this out. Every time I hear someone complain that this Arizona law is defining pregnancy as starting before conception, I start grinding my teeth and occasionally biting.

  6. Gib

    When my wife was pregnant, and the doctors, sonographers, baby books were all speaking of the pregnancy weeks as if it was from her last period it drove me CRAZY! At every opportunity when speaking to a doctor or someone and the question of how pregnant she was came up, I’d calculate from estimated conception, and put in some snarky comment about how they probably meant that silly measurement which was an extra 2 weeks.

    I finally annoyed my wife so much she told me to shut the hell up and learn to live with it. So, I pretended to, and now with our second child I’m using the silly measurement, but feel dirty inside……………

    I realise I’m a little bit crazy/OCD, but meh….

  7. 7

    “In South Dakota, physicians are required to tell women seeking abortions that the procedure causes an increased risk of suicide, according to a new law.”

    Yeah, like being gay ’causes’ an increased risk of suicide too.

    After struggling to scrape together the funds fast enough before you’ve passed an inane developmental deadline, traveling what may be hours away, and jumping through whatever the latest legislative hoops are, then you get the added bonus of having to run a gauntlet of in-person guilt bombardment from slut-shaming fanatics to enter and leave the medical facility, and likely some follow-up doses afterward from anyone in your life who caught on to your actions.

  8. 9

    Doctors do not define pregnancy as being from the first day of the last menstrual period at all. It is a means of DATING the pregnancy to get a more accurate idea of the age of the pregnancy, and is nothing more than a prediction. It’s a pretty poor one at that as only about 10% deliver on the due date, and these days is superseded by more accurate estimations of gestational age via ultrasound. When biologically pregnancy starts is a red herring here, the woman is pregnant and what is needed is some estimation of gestation of the foetus, nothing else.

    The Arizona then misused this DATING method to define pregnancy gestation for the purposes of their law, instead of the method they should have used (that of gestational age). They meant to do that, as that means that if the 18-20 week anatomical scan (which generally would be dated using the LMP method) came up with foetal abnormalities, the woman and her doctor can’t do a thing about it. They surely would have been aware most terminations of pregnancy are much earlier, and later ones are generally done for a reason related to foetal or maternal health.

  9. 10

    I see they are arguing “the law improves upon informed-consent regulations for women seeking such procedures, while protecting women and fetuses.” Stop with the protecting, and start treating women as adults who can make their own decisions!

  10. 11

    When my wife was pregnant, the doctor counted from her last period and declared that our daughter would be born on such-and-such a date. I pointed out that she couldn’t have conceived on that date, and the baby would be due a week later. The doctor promptly dismissed me as an ignorant father-to-be, but I was off by a day, whereas he was off by eight.

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