The problem with LMP-Based Pregnancy Calculations, Obstetricians and Arizona

Look, I’m not actually pro-choice. I wouldn’t bother to mention it, except that many of you follow my posts and might feel like I was a big huge liar if I suddenly started commenting on an abortion issue from a pro-choice argument. Not to be wishywashy, but for those of you who don’t know me, I’m not in that grey “I wouldn’t do it, but you can” area either. I’m talking totally left field. My personal beliefs on abortion fall much more in line with ancient midwifery practices.


My stance on the abortion issue has absolutely nothing to do with this post, so I won’t bother to delve into it further.

Arizona has passed a new law concerning abortions. I’m sure you’ve heard of it. Here’s the text of it.

Here’s the part that has gotten women everywhere, including many in the pro-life (or even left field) camp pretty riled up:

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

So, according to this law, a woman’s window to obtain an abortion just lost at least two weeks.  Now, I really do get why they did it. They want to use the same “week schedule” that doctors use.  They don’t want women told by Planned Parenthood that they are 4 weeks pregnant to do a google search for “four weeks pregnant” to find a barely recognizable fetus that is only 2 weeks old. Perhaps a different photo of a fetus at the actual stage of the one she is carrying would alter her decision. I get that. But this new verbage in the Arizona abortion law has enormous consequences.

Not all women ovulate two weeks after the start of their last menstrual period (LMP). Women’s bodies don’t work that way. I mean sure, often they do, but often they don’t. Some women won’t ovulate an egg to be fertilized for six to eight weeks after their last period. Some women will ovulate twice in one cycle. Our bodies vary. Our bodies aren’t machines.

I know that when women are pregnant it’s exciting to say, “I’m four weeks,” but we’re not really fooling ourselves. Lord knows, our husbands or lovers get awfully confused by it. How many young men do you think have seen the paperwork stating the due date have protested that they are not the father because forty weeks earlier was not “the night?”

We shouldn’t be making laws stating, “Gestational age means the age of the unborn child as calculated from the first day of the last menstrual period of the pregnant woman.” Instead of changing abortion laws to reflect obstetric terminology, we should be changing obstetric terminology to reflect the actual age of the unborn baby. From there, the lawmakers can debate on how far along a pregnancy is allowed to be for a woman to be allowed an abortion.

We have pretty accurate ways to judge the age of an unborn child. We have amazing ultrasound technology. Even fundal height would be a more accurate way of determining gestation than last menstrual period.

This LPM-based system causes havoc already. LMP-based calculations result in preterm deliveries being induced. LMP- based calculations result in unwarranted tests. LMP-based calculations result in membranes being stripped too soon. LMP-based calculations result in significant complications in a lot of pregnancies. So, why are we building upon a broken system?






  1. Great article. Its unfortunate that we have doctors even making decision for women and their bodies. I have found, as a midwife, that fundal height measurements tend to be more accurate than even ultrasound. Thanks for sharing your insights.

  2. Jody Pontius

    I must say, I find this rather humorous indeed! When I became pregnant with my 2nd child, my son was 23 months old. My LMP was right before I became pregnant with HIM. (he nursed on-demand and had a lot of strength in his mouth and we co-slept which is probably why my cycle had not ‘returned’)/ However, going by their criteria, I’d have been 32 – 33 MONTHS along at the start of my pregnancy.

  3. Lauren Rachel

    My cycles are 35-36 days long so my LMP makes my ‘due date’ off by a full week! Thankfully I did ovulation temping and knew when I had ovulated/conceived and my OB trusted that and changed the date. but with BOTH pregnancies I had the nurse who originally took my info tell me “that doesn’t matter” and consequently give me the incorrect gestational age. Very thankful to have a smart OB who pays attention to his patients!
    but yes- this law builds on a broken premise- LMP based calculations are probably often incorrect (unless you have a perfect 28 day cycle!)

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