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Dating your pregnancy…why LMP isn’t terribly effective for many women

I’m in the midst of my own pregnancy dating quandry, so I thought I would share with you why dating your pregnancy is largely a useless excercise as measured by LMP (or last menstrual period).

LMP dating is based on the idea that a woman has a 28 day cycle and ovulates on day 14.  Which might be true of some women (some of you must be the statistical average, right?), but is often not true of most of us.  Some of us have shorter or longer cycles and many of us don’t have the perfect 14 days before you bleed ovulation.

There are, of course, ways to know when you ovulated.

  • You can do Basal Body Temping and keeping an eye on your cervical mucus You really need to do both methods as temp charting will only alert you to ovulation after it has occurred, which is impractical as you want to have having a lot of sex for the five days preceding ovulation.
  • You can use the pee-stick ovulation predictor kits These alert you to the hormone surge when you hit the luteal phase, giving you roughly 48 hours of notice (although not always 48..sometimes as little as 24)
  • You can keep track of your periods over time and get a sense of what your “normal” cycle is, and subtract 14 (imperfect, but close) and then target your get pregnant sex to 5 days prior and 2 days post that magic 14.  (there are plenty of period apps if you have a smartphone, or you can just use a calendar).
  • If you keep an eye on the pattern of your sexual drive, you may also get an idea of when you actually ovulate, if you (like me) have a sex drive that correlates to your ovulation (again, use this in conjunction with a calendar or period app).

Obviously this works better for those of us who were trying to get pregnant, and were keeping track of their period.

My cycle is usually 35 days with ovulation likely occurring on day 21/22.

Of course, the period that I got pregnant with broke all the rules.  I was on day 48/49 of my cycle before I got a damn positive…2 weeks later than I SHOULD have been able to get a positive.  Looking at my sex drive and the frequency of our sex, I had already theorized that I ovulated later than normal.

I had guesstimated that I was 5 weeks pregnant last week, but my hormone levels and ultrasound results gave me some doubts that were further put into doubt this week.  I showed progression, but progression that put me about a week behind my guess (as did last week’s results…giving me consistent positive progress, but not on pace with where I expected to be).

So how do you discover the baby’s “real” due date?

Early ultrasound is the best technique.  Until the end of the first trimester, babies develop at a predictable rate of development, and dating is relatively easy.  The problem is that we are talking about ridiculously small measurements, where a mistake can mean a dating error.  It’s best to have 2-3 early dating ultrasounds that corroborate progress and infer your due date from that.  Easily done in a country like Singapore, but not so in the US.

Why is dating so important?  If you’re looking at a C, or a scheduled induction because of a complication like diabetes (we tend to experience placental breakdown post 40 weeks, making it dangerous for us to go over full term), you want to make certain that you’re not being induced too early (which could mean the baby’s lungs are not yet fully developed) or too late (if you have risk factors).  The good news is that high risk women tend to get more ultrasounds under American insurance, which helps with dating.

If you’re low risk, it’s not a big deal to go a few weeks “late” (which you might not be).

So take that EDD (estimated due date) with a grain of salt.  If you know your body, you may be able to better calculate your due date than any of those silly circles our OBs like to use.

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