Post-term pregnancy: what is the clinical evidence?
-by Rachel
Westfall, 2002. View Rachel's other
works on BirthLove, linked from the bottom of
this page. Visit Rachel's site.
Over the past hundred years,(1) medical opinions
have shifted regarding the appropriate length
of human gestation. Do babies come when they are
ready, or do some need help in order to be born
at their optimal time? Is there a really an optimal
time,(2) and how large is the window? What is
the appropriate length of gestation, anyway? Is
it indeed 280 days from the first day of the last
menstrual period, as presently calculated, or
should we begin counting from the end of the last
menstrual period?(3) Are we in fact sure of the
pregnancy is prolonged, or was the date of conception
calculated incorrectly?(4-8)
Obstetricians and medical researchers have written
extensively on the problems that can be associated
with childbirth, and they have reported a greater
prevalence of some of these problems in babies
who have had longer than average periods of gestation.
Such problems include wasting of the unborn child
due to placental degradation, described at length
by Clifford (9) and Vorherr (10) and since debunked.(11)
There are concerns about post-maturity syndrome,
which is typified by peeling skin and reduced
body fat, though its name is perhaps a misnomer
since it is not only seen in postdates babies,(12)
and is in fact more frequently seen in term babies,(13)
though it affects a higher proportion of postterm
babies.(14) Other concerns include macrosomia
(large size) (11) and higher incidence of neonatal
asphyxia and meconium aspiration syndrome.(13)
Indeed, some older data (15) suggest that the
rate of perinatal mortality climbs rapidly with
prolonged gestation, such that it has "doubled
by 43, more than tripled by 44, and more than
quintrupled by 45 weeks."(15) However, no
modern data have demonstrated this relationship
to the same degree.(16-22)
Attempts to manipulate the data to replicate
McClure Browne's classic U-shaped curve of perinatal
mortality rates (15) by basing the calculation
on numbers of ongoing pregnancies (16) have been
less than convincing, for they fail to show the
increased risks associated with very premature
birth. "The higher risk that routine induction
at 41 weeks aims to reduce is dubious, if it exists
at all." (23) Regardless, it is McClure Browne's
long-expired data that are frequently dragged
out, brushed off, and presented as evidence for
the dangers of prolonged gestation.(24-27)
In making the connection between higher rates
of perinatal complications and prolonged gestation,
some obstetricians (28) have implicitly and explicitly
forwarded a program of routine induction of labour
for all mother-babies who go beyond a chosen gestation
period, often 41 or 42 weeks. As a result, "the
standard of care in Canada is now assumed to be
routine induction at 41 weeks."(23) This
is in spite of the fact that there is no conclusive
evidence that the relationship between prolonged
gestation and higher rates of perinatal complications
is causal.(29) In fact, the connection between
postdatism and higher rates of complications is
itself suspect.(23,30)
An additional concern is that routine induction
of labour- at any stage in gestation- has not
been conclusively shown to improve outcomes. A
number of studies have shown better outcomes for
mother-babies who are left to go into the birth
process naturally, with fewer complications and
interventions such as Cesarean section (6,22,31-40)
or no difference in outcomes either way.(41-44)
In a few studies, induction resulted in lower
rates of Cesarean section and other complications
than expectant management,(28,45-48) but the largest
of these studies (28) has been sharply criticized
due to methodological and analytical flaws.(23,49)
"I think we must draw a much more limited
conclusion: Women who have the misfortune to be
managed by knife-happy obstetricians may be somewhat
better off having an induction if they exceed
their estimated due date. More than that, we cannot
say."(49) Regardless, prolonged gestation
continues to be the most common indication for
induction of labour.(5)
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