Pregnancy Wheels and Due Dates

The first day of Normal Prenatal was this week. We learned a ton of stuff, and then we pulled out our pregnancy wheels to practice with due dates, irregular cycles, etc. In our relatively large class, we had a really wide range of dates, as many as 4 or 5 days depending on how each different pregnancy wheel was constructed. And of course, that got me thinking.

Due dates are such arbitrary things. First of all, they are calculated not on the date of ovulation and therefore conception, but on the date of the last menstrual period. This of course is based on the assumption that there is such a thing as a “normal” woman with a perfect 28-day cycle. Even when a woman knows for a fact which day she got pregnant, it does not change the way the due date is calculated.

A due date is such a simple thing, really.  A guess at when the baby might be born. But little things like inaccurate due dates add up and sometimes it becomes incredibly important. When women are told (as they all too often are) that it is dangerous to go past their due dates and they must be induced (because the placenta might break down!) or when women are routinely induced at 38 weeks (so that the baby won’t grow too big to fit!), an extra 4 of 5 days could have made a world of difference.

I predict that I will continue to have these sorts of completely obvious epiphanies (now, with added depth and sparkle!) as the weeks go on. It’s all of that work of rounding me out as a midwifery student.


5 responses to “Pregnancy Wheels and Due Dates

  1. I’ve always been interested by this method of due date calculation. I just fail to see how it makes any sense. I’ve never in my life had a cycle that was shorter than 32 days. A specific due date just seems like a symptom of our overly scheduled lives. Goddess forbid, Baby wants to bake a bit longer when we not only have X,Y, and Z to do but also A through W.

    Glad to hear things are epiphany producing!

  2. Scary that the wheels can be 5 days different. Say you may be off anyway on your date a little bit, and then the pregnancy wheel is five days off. So you may be at least a week off of the actual gestational age. Then you take the quad screen at 16 weeks…but you’re told you have a high risk for T-18 or Down’s. This is all based on dates. You go on in for sonograms that show baby is a week different or more on size. Now you have a marker for these disorders. Then you get amnio, which has a risk of miscarriage. This is unacceptable. My doctor used a wheel and had the day one off from my due date by my calculation (using 40 weeks from LMP) but at least 5 days off of ovulation by my calculation. My cycles are also around 35 days long. The sonogram gave a date later than the doc did on that wheel. I’ve tested positive for Down’s on a penta screen at the supposed 16 week date. Now I’m having another sonogram, and we’ve got extra worry and extra monitoring. I don’t want to have my baby at a different hospital though I hated the nurse’s treatment last time because it’s the one with the best NICU in case I do have a baby affected. This could all be a mistake due to the pregnancy wheel and miscalculated due dates. Ugh.

    • Exactly why all of this is so important. I hope there are more accurate pregnancy wheels than the cheap one I’ve been using (just for practice, not to actually calculate someone’s due date). Because these are exactly the type of situations I’m talking about. I wish you the best possible outcome with your current testing procedures.

  3. This is why I always make a point to tell my OB, at my first appointment, “I believe my due date is off as I ovulated on or about such-n-such date” and they either trust me and change the due date or we get a sonogram to date my pregnancy and you know what? I’ve always been spot on about when I felt my due date was off. My last pregnancy it was off by TWO WEEKS and it felt great to know how well I knew my body.

  4. You know, Joy, I actually picked my baby’s birth to the exact day last birth. I felt the initial sonogram even was off. I kept getting later sonograms showing later due date, but know my children are short. I looked at my calendar, looked at the evidence, and picked the birthday exactly. My OB was talking induction when I hit her 40 week day, but I argued hard and had my baby right in time.

    This time I really believe we are about 5 days off or more…by ovulation and longer cycles. I saw the initial dating sonogram, it was not very clear and she didn’t even see my ovaries on there. I think the dating is a bit off…I really do.

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