How is Gestational Age Calculated?

Gabriel at 8 weeks past LMP (6 weeks after conception)
No, Arizona did not pass a bill that says a baby's life begins two weeks before conception. That is what's known as "maliciously false pro-abortion propaganda."

Here is the text that has everyone up in arms, from HB 2036:

4. "Gestational age" means the age of the unborn child as calculated from the first day of the last menstrual period of the pregnant woman.

I've been pregnant six times, and every care provider I've ever seen (two different OBs and a midwife, in two different states) has calculated pregnancy this way. In fact, with Gabe it was a bit of an issue because I insist on calculating my pregnancies based on conception date since I have long cycles, and the medical assistant at my OB's office only knew how to calculate gestation based on LMP (last menstrual period). But hey, don't take my word for it:

  • Calculating the day your baby begins to develop and keeping track of your pregnancy dates can be a challenge. The development of pregnancy is counted from the first day of the woman's last normal menstrual period (LMP), even though the development of the fetus does not begin until conception, which is about two weeks later. - American Pregnancy Association

  • In the UK pregnancy is calculated from the first day of the woman's last period so for as much as three weeks of this first month she might not be actually pregnant. - BBC News

  • The average length of a pregnancy is 40 weeks, or 280 days, from the first day of the last normal menstrual period. - Discovery Health

  • Pregnancy is measured using “gestational age.” Gestational age starts on the first day of a woman’s last menstrual period (LMP). - Planned Parenthood

Yes, that's right - Planned Parenthood determines gestational age in the exact same manner as stated in HB 2036! Bet HuffPo missed that little factoid.


  1. Oh good grief!!!!! The only word I have for the liberal media is *facepalm*

  2. Seriously? Could they be bigger morons? Sorry. But it's true. They're grasping at straws. Oh wait, that means we're winning!

  3. 'Cause these people have NEVER BEEN PREGNANT. They have zero idea what they're talking about. It's crazy!

  4. Hi JoAnna, came here from Devin's blog (you always post such good comments!) This post speaks to me. I ended up switching OBs (actually I went to a midwifery practice) when I was pregnant with my son (so far the only one) after enduring a lot of stupid crap, but it really frustrated me that they insisted on such an early due date when I *knew* it wasn't accurate (I have long cycles, or had...still in lactational amenhorrea so we'll see what happens when my cycles come back). I just had to mention my own charting to my midwife and they revised the due date. Good thing, too, because I probably would have had to have labor induced otherwise!

    I agree with Dwija that a lot of the hoopla over this is because people haven't been pregnant and just have no idea what it's like (depictions of pregnancy and childbirth are not exactly positive in mainstream media). I got into an argument with a woman once who insisted that all pregnant women be given free voluntary C-sections regardless of medical need because vaginal delivery was "just too traumatic." This woman has never been pregnant and my son was around two months old at the time. Weird, weird stuff.

    1. Oh, I hear you, Silica. With my last pregnancy I ovulated on something like day 25, so my due date would have been very inaccurate if they'd calculated it according to LMP.

      I also don't get the insistence that major abdominal surgery is less traumatic than vaginal birth. I've had four unmedicated vaginal births and I much prefer them to having major surgery!

    2. I'm hoping for unmedicated if there is a next time, God willing. Tried to the last time but after 45 hours my midwife, quite rightly, suggested Pitocin to give DS the extra help he needed coming down, and it worked, and he was fine! 52 hours of labor total, and I still would take it over a C-section (that I probably would have had much sooner with a standard OB.)

      My midwife did joke with me at my postpartum checkup that any future babies will most likely seem to "fall out" in comparison! I'm not scarred by the experience. It was work - they don't call it labor for nothing - but it wasn't traumatic. Getting my wisdom teeth out was traumatic. Still get the shivers when I think about it.

  5. "I also don't get the insistence that major abdominal surgery is less traumatic than vaginal birth."

    Who says this?!?!?!?!?!?!?! I've delivered 5 children, vaginally, and while I had an epidural with every one, they were all FAR less traumatic than the the three times I've been under general anesthesia in an OR - and NONE of them were considered major surgery, one surgery two reset a broken arm, two on my left knee. And for the record, my first child was born 5 1/2 weeks early after I had been in the hospital for 7 days with toxemia. It was still less traumatic than somebody cutting my knee open to realign my knee-cap and remove the piece of bone from said knee-cap that chipped off a few years prior.

    1. The D&C I had for a missed miscarriage was more traumatic for me than any of my vaginal births. Physically, the births were more taxing, but not extremely so.

  6. It's so frustrating when you're at the hospital and you try to explain how the pregnancy is dated (using NFP) and they look at you like your crazy. When I was in for spotting a few weeks ago the ultrasound tech told me she wished people would stop trying to go by when you conceived and just use the LMP (I have long cycles too). I didn't point out that by my date the baby was only 4 days off (and when my OB did it was only 1 day off). By the LMP the date was like two weeks off and we would have been panicking not to have a heartbeat at 7 weeks instead of 5.

  7. Just chiming in to say that while vaginal delivery is prefered, I had a c-section recently (no vaginal births yet, but I'm hoping!) and thought recovery was a piece of cake. Pain was minimal and there were no stitches/staples/wound dressings. I just had a lift restiction which required more time off from work (I a very physical job). I know not everyone gets so lucky and that it is major surgery--however, I don't think it was as bad as some make it out.


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