From: sfm@manduca.neurobio.arizona.edu (Stephen Matheson)
Newsgroups: talk.abortion
Subject: Re: FETAL AGE ESTIMATES (with apology to LM about LMP)
Date: 26 Jan 93 02:39:29 GMT
Organization: University of Arizona UNIX Users Group

From article <nyikos.727994815@milo.math.scarolina.edu>,
by nyikos@math.scarolina.edu (Peter Nyikos):

Odd  number of >'s = Peter Nyikos
Even  "     "  "   = Larry Margolis

[Massive deletia.  Neither man is lying.  Case closed.]

>>>One little nagging point: could you tell me where I can look up these
>>>stats myself?  I want to make sure the expression LMP was used.

>>The following:
>># 50% of the 1.6 million abortions each year take place at 8 weeks or less
>># from the last time the woman menstruated (LMP); 27% at 9-10 weeks LMP; and
>># 12% at 11-12 weeks LMP.  6% of abortions take place at 13-15 weeks LMP;
>># 4% at 16-20 weeks LMP; and less than 1% at 21 weeks LMP or more (0.6%).

>>is a direct quote from "Facts in Brief / Abortion in the United States",
>>published by The Alan Guttmacher Institute.  I'm the one who typed it in
>>originally, and I just double-checked it against the paper.  If you
>>doubt my word, you can write them and ask for a copy:

[addresses deleted]

>>>>>No, Larry, there are ways of estimating fetal age besides the woman's
>>>>>word, and if you don't know that, your knowledge of abortion is truly
>>>>>Lilliputian.

>>>>That's estimating gestational age, not estimating time since LMP.

>>>I wonder whether "gestational age" refers to time since implantation.

>>My dictionary refers to time since conception, but I admit that I don't
>>know if this is the same definition the medical references use.  (An
>>interesting question, but unrelated to the above, since LMP is what
>>was used in the statistics given, not gestational age.)

> Nowadays conception is taken to be synonymous with implantation.

> "Conception is the implantation of the blastocyst.  It is not synonymous
> with fertilization."  E. Hughes, ed. _OB & GYN Terminology_, Philadelphia,
> F. A. Davis, 1972. 

>>>it is customary to say that pregnancy starts with implantation, not
>>>fertilization.

Okay, we have a real problem here.  I am quite convinced that gestation
begins at fertilization, and I have always used conception as a synonym 
for fertilization.  I can think of simple ways to estimate the approximate
time of fertilization.  I can think of obvious ways to ascertain time
of LMP.  I'm not sure how we can know implantation time without first
knowing something about fertilization.  So, until I discover otherwise,
I will assume that we measure pregnancy from fertilization to parturition
(38 weeks) or from LMP to parturition (40 weeks).

I realize now that the term "conception" contains unintended ambiguity.
Great.

>>Actually, I think it's more customary to refer to when the sex act took
>>place, rather than to either implantation *or* fertilization.  ("Do you know
>>when you got pregnant?"  "It was the night we went on the hay ride.")

> Seems like there is a little inconsistency in the preceding few paragraphs.

Fertilization could follow the hay ride-associated activities by many hours.
Is that what you meant, Peter?

[deleted bickering]

>>>I merely shed doubt on the correctness of yours.  As I do below.
>>[...]
>>>In fact, Hern writes in _Abortion Practice_, copyright 1984:
>>>
>>>	Correlation between the sonographic estimate of fetal age
>>>	and the measured result is 0.97...whereas correlation with
>>>	the last menstrual period is only 0.62 at best and the correlation
>>>	with the physician's estimate is 0.87.  [pp. 209-210]

>>I don't see the point, here.  LMP is Last Menstrual Period; it's obviously
>>a way of *bracketing* when conception occurred, not pinpointing it exactly.
>>Stephen Matheson took this into account when doing his calculations.

> But part of the issue is how often the woman's recollection of LMP is  
> valid.

[deleted tangential tussle]

>>>>Are you suggesting that a sizable percentage of women don't know when
>>>>their LMP was?

>>>Seems like Warren Hern does that above.

The inaccuracy of the LMP estimation method must be due in part to
inaccurate recollection of the date of the LMP.  But I wonder if this
is the major contributor: we all know that menstrual cycles can have
considerable built-in variability...

>>That doesn't answer the question.  First, saying that using an estimation
>>with a month's leeway doesn't correlate well with something that attempts to
>>pinpoint the exact age says nothing about whether the woman's statement of
>>when the *reference point* for the first estimation occurred is accurate.

A month's leeway?!  Let's give LMP a little more credit than that...

> A month's leeway makes hash out of all claims that most abortions occur
> in the embryonic stage.

True.  This discussion would be a lot more interesting if we had histograms
instead of arbitrarily-chosen categories of time since LMP.

--

Steve Matheson   Program in Neuroscience   University of Arizona
sfm@neurobio.arizona.edu