It should be a simple, easy answer, shouldn’t it? But it’s not. It’s complicated by tradition as well as politics.
Some time ago, I was arguing on an abortion blog, and some guy said something about, “You pro-lifers say that pregnancy begins much earlier than doctors do!” Well, he’s sort of right but mostly wrong.
Many people, myself included, are strongly pro-life, and count life to begin at conception — which is when embryology textbooks also say is the beginning of human development. Before the mid-1960s, conception was generally considered to be the start of pregnancy, but with the politics of abortion looming large (not to mention birth control pills), the medical community changed the term, saying that since we didn’t really know for sure when conception took place, pregnancy shouldn’t be considered to have begun until the fertilized egg actually implants in the lining of the uterus. This allowed doctors to prescribe drugs and hormones which would interfere with implantation, but not to break laws against abortion. It also allowed pro-life doctors and pharmacists in good conscience to prescribe and dispense hormones which might allow ovulation, but have as a “back-up mechanism” keeping the blastocyst from implanting. Many people (such as myself) are uncomfortable with this, and would consider it to be abortifacient. Technically, it’s not, because it’s not considered to be an abortion until the blastocyst has implanted and develops into an embryo.
So, that was the guy’s point — that we put the start of pregnancy back at conception, which is up to a week before implantation, which is when ACOG declares pregnancy to officially begin.
But it’s not as simple as that. Because in dating a pregnancy, and saying that your due date is at 40 weeks, with term being anywhere from 37-42 weeks, they’re not dating that from implantation, nor even conception. Rather, pregnancy officially starts on the first day of your last menstrual period (LMP), which is usually about 2 weeks before ovulation. So, on one hand, your pregnancy is considered to have started two weeks before even the most ardent pro-lifers will say that pregnancy begins, and about three weeks before implantation, which is when “pregnancy” is defined as starting, for purposes of labeling something an abortifacient.
Why? A German obstetrician back in the 1800s decided that since women’s menstrual cycles occur monthly like lunar cycles, on a 28-day cycle (average), then pregnancy should take ten “lunar cycles”, or ten periods of four weeks, or 40 weeks. But there are two problems with this — first, 40 weeks from conception would make the average or typical pregnancy last to what we currently call 42 weeks. While many women would naturally go this long (and most without a problem), most women would have given birth by this time. The second problem is, that ovulation is not necessarily a memorable or noticeable event, but menstruation certainly is. So, presto change-o, pushing the start of pregnancy back to two weeks before conception, or the start of a menstrual period, solves both problems. While keeping intact the precious, preconceived notion [pun not intended] and dogmatic assertion of a German doctor. Above all else, unsubstantiated claims must be kept intact, despite actual evidence to the contrary, as long as a doctor came up with it!
Yes, I’m being sarcastic. 😉
This makes things a little difficult, sometimes, when thinking about stuff that happens in the early days of pregnancy — some books, websites, etc., will date the pregnancy as starting at conception, while others will date it starting at LMP. So, whenever I hear something about embryo development, and things like “the heart starts beating at 5 weeks,” I wonder, is that 5 weeks from conception, or 3 weeks from conception? After you get out of the first trimester, it gets easier, because it’s pretty standard to switch to “gestational age” (i.e., from LMP) in full. I remember one book I read when I first was pregnant that was a pregnancy picture book, and they did this — in the first trimester, the pictures were all dated from conception; but in the second trimester, the pictures were dated from LMP. I wouldn’t have noticed, except they drew attention to it; I suppose as a way to avoid having the first two weeks worth of pictures being blank? I always kinda wondered, where did those two weeks go? 🙂
Also, as an aside, I know at least one woman whose pregnancy officially started when she was still a virgin, thanks to pregnancy starting on her LMP, a week or two before her wedding, and she got pregnant on or soon after her honeymoon. It’s things like that that make my “OCD meter” go haywire! If we’re going to put things in nice, neat little packages, let’s make sure they fit, okay?