Overdue or Simply Owing to Innocuous Abnormality?

I’m breaking my own moratorium on prenatal blog posts due to some new information I got from the hospital today when I asked for a fetal non-stress-test appointment. My homebirth midwife, Maria, had recommended I do so as soon as possible, since as the night swings us over to October 1st, I’m now exactly 1 week past my due date — 41 weeks pregnant — according to my due date of September 24th.

In the prenatal world, due dates are the all-important arbiter of progress, scheduling, health, and — most importantly — danger. Risky outcomes for fetuses reach a nadir at 38 weeks and then slowly rise every day as the pregnancy progresses. Today, 42 weeks is considered the absolute limit in length of a healthy pregnancy.

I’m actually less than convinced about the 42-week safety barrier, at least as far as my own situation is concerned. Here’s why.

The following may be classified “too much information” for some people because it’s very plain talk about sex and lady things. Proceed at your own risk and don’t come whining to me or my mom about something icky you accidentally read. Yeah, you heard me.

There are several different methods of calculating due date based on some kind of normative averages for women’s bodies, i.e. length of menstrual cycle (28 days) and length of luteal phase (14 days).

The rough and dirty standard for a due date is based on the first day of your last period before you got pregnant. Then it factors a 28-day menstrual cycle to decide when you ovulated, figures more or less immediate implantation, and then adds 280 days. Poof, that’s when the baby’s going to pop out.

The problems with this can be described abstractly, but I’ll just use myself as a prime example. Try using CPMC’s Due Date Calculator using December 1st, 2009, as the input date.

According to that, I conceived around December 15th and I am today 3 weeks and 2 days overdue. GASP!

The only problem with that is that I wasn’t having sex anytime around that date. The next sexual contact I had was on December 31st. Or January 1st, depending on how you look at it.

Ok. Given that my next period definitely hadn’t started yet, that means I’m WAY off the average for menstrual cycle length and possibly also for luteal phase length.

And indeed, that is the case. Although I’ve never seen fit to calculate my luteal phase, so I don’t know it, my menstrual cycle falls in a range between 30 and 34 days of length, depending on how much exercise I’ve had, how much stress has been in my life, how much sleep I’ve gotten, and how much fat was in my diet. On January 1st, I would have expected to see my period start that day or the next.

But it didn’t. Ahem. We have a conception date.

Playing with this due date calculator that allows adjustment of menstrual cycle and luteal cycle lengths shows me that, using this model of fertility, the only way I could have conceived on January 1st would be if I had a cycle length of 45 days. But I’d been pretty regular all last year, despite lots of changes in my life, and 45 days is a ridiculously LONG cycle, besides.

The more plausible scenario is that I was still fertile on the last day(s) of my cycle. That’s not accounted for in the standard model, so we just have to move on to something that accommodates more abnormality.

(Let me just pause to marvel at how unlikely it is for anyone to conceive at the tail end of a cycle. In fact, the statistical likelihood of George’s conception goes down with each detail one considers in the whole story [which 99.99% of you will never hear]. Someday I’ll put a real number on it, and it will be devilishly small and adorable and precious… I digress.)

Due date calculators that start from conception date are simple date-additions (conception date plus 266 days), but they do a much better job of predicting gestation length than the cycle/luteal calculator. So let’s feed January 1, 2010, into the calculator for conception date, yielding September 24th, 2010, as the due date.

Done! I’m due September 24th. I need to deliver on or before October 7th, 2010, to stay within the “safe zone” of 42 weeks for gestation length.

There’s just this one huge hitch. I think I’m predisposed to a 43-week gestation length.

My midwife won’t let me go past 42 weeks, she says, and St. Luke’s Hospital won’t even accept me as a patient past 41 weeks, 5 days.

I’ve been playing along with the whole “42 weeks or bust” thing, thinking it wouldn’t be an issue — of course I’ll deliver “on time”! — but now I’m getting worried.

I do not want to be induced with pitocin at the hospital and subjected to the whole cascade of interventions that will impede or totally obstruct my ability to have a natural experience where I am engaged and feeling every part of the birth. And I certainly am not cool with the increased chances for a caesarian section that come along with the interventions.

It’s really upsetting to me that I could be subject to all of that simply because my little patch of normal and healthy operation lies on the freaky outskirts of what is statistically considered normal and healthy based on global averages.

This story isn’t over, though. The phone call from the hospital this afternoon changed my outlook. Next blog post.

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