Why Don’t They Predict That?

In a comment on the FutureBaby betting thread, Rivka writes:

(I’m laughing at all these people who are predicting early. The average gestation for a first time mother is 41 weeks, 1 day.)

Of course, as a physical scientist, my immediate reaction to that is “If they know that, then shouldn’t that be figured into the projected delivery date?” It’s not, though– we can count up the weeks, and the projected date is at exactly forty weeks.

So why doesn’t the projected delivery date reflect the average gestation time for a first-time mother? As someone who has taught pre-med physics, I have an extremely cynical and unflattering guess, but I’d love to hear an actual explanation.

Comments

  1. #1 Evan Goer
    July 14, 2008

    It’s literally because some guy in the 19th century came up with a nice round number, and nobody bothered to check his figures until nearly a century and a half later.

    http://transitiontoparenthood.com/ttp/birthed/duedates.htm

    Hospitals still promulgate the “40 weeks” date because it gives them an “argument” to induce babies at 41 weeks, even though that’s the average. If you can induce more babies, your staffing requirements become much more predictable.

  2. #2 mollishka
    July 14, 2008

    Of course, these things also depend on the mother’s age, which is another tidbit of information we weren’t provided with. (And then, as someone who was born 5 weeks early to a 1st time mother, I find this whole “predicting” thing rather amusing to begin with…)

  3. #3 Kate Nepveu
    July 14, 2008

    these things also depend on the mother’s age

    Huh, I didn’t know that.

    I’ll be 31 quite soon.

    And yeah, of course this is for amusement purposes only . . . my niece, a first-born, was six weeks early.

  4. #4 Uncle Al
    July 14, 2008

    Birth occurs when the fetus outgrows local resources and begins to starve. Big dad, petite mom. Early delivery is a defensible prognosis.

    One wonders whether modestly reducing a due mother’s blood sugar concentration would displace 100 years of medical “wisdom”. Western birth posture is certainly the work of an idiot.

  5. #5 mollishka
    July 14, 2008

    Yup. The older the mother, the more likely the baby is to be premature:

    A 2005 government study found that women over age 40 were 40 percent more likely than younger women to deliver prematurely (before 37 weeks of pregnancy) (6). … Other studies have found that women between 35 and 39 also have an increased risk of premature delivery, though their risk may be lower than that of women over age 40 (7).

    But at 31 this is all irrelevant.

  6. #6 Honu-girl
    July 14, 2008

    That 41 weeks, 1 day, is for first time CAUCASIAN moms, IIRC. Asians and African-Americans have different averages, too, both less than 40 weeks, IIRC. (Excuse my faulty memory, though, as with two small kids my IQ has significantly lowered.) So, many, many factors come into play – age, first time mom, race, general health, and the baby’s temperament :D.

    First child, born when I was 30, arrived 3.5 weeks early. Second child, born when I was 33, arrived 5 weeks early. Both “preemies” technically, but no problems (in fact, my first-born was pretty big, even early).

    As a Caucasian woman, I fully expected to be “late.” Surprised me that both kids came early, and so early at that – the baby comes when the baby is ready to do so.

    Not much consolation in these final days, I know. But know that the baby IS coming :D

  7. #7 tceisele
    July 14, 2008

    You know, something that has been bothering me ever since my first daughter was born, is that for something that people have been doing for all of our existence, the doctors seem to be awfully willing to proceed blindly without even checking to see if the most common problems might be cropping up. In particular, there is a problem when the baby is facing the wrong way at birth: if it is coming out facing forwards, the delivery is far more painful, difficult, and more likely to result in getting stuck than if the baby is facing backwards. This is not rare: it happens maybe one time in six (and it happened to my wife both times). And yet, do they make any effort whatsoever to determine which way the baby is facing at the start of labor? NO! They do NOT! I mean, here they are, they’ve even got a portable ultrasound imager in the fool delivery room, and they stand there and bald-faced tell me that they have no way of finding out how the baby is oriented until it is too late. And they show no sign of even caring! “Oh, that’s just the way it is sometimes”, they say. Bah. At least, after the doctor and nurses finished spending about ten hours berating my wife for being a wimp, and then finally decided that maybe things weren’t proceeding well and a C-section was warranted, the doctor had the decency to concede that the baby had been badly stuck and to apologise to her for putting her through that.

    With that sort of thing going on, I am not surprised at all that they haven’t worked out a reasonable way to predict birth dates. Especially since both of ours were nominally 2 weeks early, and yet the doctors kept saying about each one “That’s funny, she looks full-term”.

  8. #8 Mary Kay
    July 14, 2008

    Human beings being the infinitely variable creatures that they are, all things will very. But it says right there on the first page of the How to be an Authoritative God-like Figure Manual that you must always sound sure of yourself. So 40 weeks it is!

    And while I at first thought the combination of big dad and petite mom could argue for an early birth, I remembered my own. I was a first child and was 2 weeks late and weighed 9lbs 6 oz. The word petite was invented to describe my mother. (No it wasn’t a difficult birth — barely 8 hours labor. Apparently the inside doesn’t match the outside.)

    MKK

  9. #9 Mary Kay
    July 14, 2008

    Human beings being the infinitely variable creatures that they are, all things will very. But it says right there on the first page of the How to be an Authoritative God-like Figure Manual that you must always sound sure of yourself. So 40 weeks it is!

    And while I at first thought the combination of big dad and petite mom could argue for an early birth, I remembered my own. I was a first child and was 2 weeks late and weighed 9lbs 6 oz. The word petite was invented to describe my mother. (No it wasn’t a difficult birth — barely 8 hours labor. Apparently the inside doesn’t match the outside.)

    MKK