A Look at Postdates Studies (and the fuzzy thinking
that accompanies them!)
-by Gail Hart
of www.midwiferyeducation.org
Certainly much of the post-dates data is difficult
to wade through because few make a distinction
between cut-off dates- in most of them a date
is picked and any baby past that date is tossed
into the statistics- meaning that a 41.5 week
baby is ranked right in there with the 43.5 and
45 week baby... This blows the stats for the group
we really need to know about- the women at 41
or 42 or even 43 weeks.
41 is very, very common. 42 is not rare at all...
but true 43 weeks is. Lumping all the post-dates
stats paints the 41 weeker at the same risk as
the post-43 weeker and leads folks to figure we
should induce all women at by 41 weeks- which
I believe is an erroneous interpretation of the
data.
There is SO MUCH GARBAGE "data" used
to support the idea of post-maturity risk. For
example, in the latest edition of "Obstetrics"
the authors refer to the danger of post-maturity
and state "after 42 weeks fetal mortality
doubles". Then they include a graph showing
3 study results of fetal mortality rates by gestation.
But if you look at the graph the fetal mortality
rate (FMR) IS ALMOST FLAT FROM 40 WEEKS TO 42
WEEKS. There is NO "doubling" of the
mortality rate! It "climbs" from 2/1000
to a tad under 3/1000 at 42. At 43 weeks it is
still under 4/1000. It does NOT "double"
at 42 weeks. The data does contradicts the statement!
They superimpose three graphs of gestational
FMR stats. One of them does climb after 42 weeks,
but it is from the early 1970 stats and shows
a climb from 6/1000 to 8/1000 at 42 weeks. (This
one shows a "doubling" at 45 weeks!)
The only graph line which shows a high rise after
41 weeks was published all the way back in 1963
from data collected in 1958 (and probably merely
shows that post-date babies were more heavily
affected than were tern babies by the then nearly-universal
use of general anesthetic). That graph shows an
abominable rate of almost 10/1000 at 40 weeks,
11/1000 at 41 and 18/1000 at 42. The only way
the authors could support their own belief about
the dangers of post-term, is to dig up statistics
from almost a half century ago!
And even though I admire anyone who has the courage
to contribute to a text-book (and the stamina
to make it all the way through medical school),
I really challenge this example of fuzzy thinking!
In order to know the truth about the risk of
post-dates, we need to be speaking of the same
definitions... and we should be looking at stats
from the same century!
The biggest risk of true post-maturity is fetal
distress in labor. Not "sudden distress",
but the tired baby who doesn't handle labor well.
Mom is a bit more at risk of needing a cesearean
since the baby may not be able to handle a prolonged
labor. (Surprisingly, the incidence of "non-reasuring
heart rate" is not as high as we have- annecdotally-
been led to believe.)
The rate of meconium staining at 43 weeks is
said to be twice as high as term, but we know
the majority of babies do NOT have meconium aspiration
syndrome- despite the state of the waters.
Yet induction of labor (especially if AROM is
done) increases the risk of fetal distress- and
meconium aspiration. By inducing by calendar instead
of "for indications", we are subjecting
babies to extra stress in labor- thus putting
all "post-dates" babies in a higher
risk status than those who enter spontaneous labor.
That single fact alone will color the statistics.
Some babies get in trouble after 43 weeks. No
argument with that... though even then it is the
minority (only 10% of 43 week kids have "post-maturity
syndrome" or "dysmaturity"). It
is certainly worth while to discover these kids-
and fundal heights and AFL estimates are probably
the easiest way to do it. But to extrapolate out
the risk at 43 weeks, and move it to 42 weeks
or to 41 weeks is not accurate.
When correct fetal surveillance is done, the
risk of postmaturity declines, along with the
induction rate and the cesarean rate. It is all
tied in together. Post-dates means we need to
watch even more carefully for signs of IUGR (intrauterine
growth retardation)- but it does not necessarily
mean we need to induce all women at some arbitrary
date
Few women's bodies function with machine-like
regularity! Many women conceive later in their
cycles than "the average", and it is
certainly true that some babies take longer to
cook than others!
-Gail Hart, Midwife, Oregon
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