Post Term Pregnancy
Vol 2. No. 9 


*Rotimi Odutayo, MD MRCOG
and **Kunle Odunsi, MD PhD MRCOG

Department of Obstetrics and Gynecology , *Queen's University, Kingston
,Ontario, Canada ,and  **Yale University School of Medicine , New Haven,
Connecticut , USA .


INTRODUCTION

It was Ballantyne in 1902[1] that first made reference to postterm
pregnancy in modern obstetrics. However, in 1954 Clifford[2] described
more succinctly a syndrome found in infants born after the expected date
of delivery which in many respects resembled intrauterine growth
retardation; thus there was often thick meconium staining of the amniotic
fluid and signs of fetal distress in labor in these postmature infants.
Auberg(1962) and Lanman(1968)[3, 4] also showed that there was an
increased risk of intrapartum death associated with prolonged pregnancy
and studies from Scandinavia confirmed that prolonged pregnancy was
associated with an increased risk of perinatal death [5]. An
observational study from Dublin examined the risks of postmaturity in
6301 pregnancies delivered at 42weeks [6]. In the postmature pregnancies,
intrapartum stillbirth was four times and neonatal death three times as
common as in the women delivered at term, and early neonatal seizures
were ten times as common. Crowley [6] also compared the outcomes of labor
in 247 women delivered after 42 weeks with 247 matched controls delivered
between 37 and 42 weeks: meconium stained amniotic fluid occurred twice
as often in the postmature women and the need for fetal blood sampling
was four times as common .

Prolonged pregnancy has gained prominence in the last decade as a
probable high-risk condition after widespread use of antenatal testing.
This notoriety has developed more as a consequence of the inability to
find the appropriate sensitive antenatal test rather than from the
acceptance of its truly life threatening condition for some fetuses. This
point is clearly observed when one reviews recent publications stating
that perinatal mortality is the same among prolonged and term gestations
[7], [8].

EPIDEMIOLOGY
The World Health Organization  (WHO)[9] and International Federation of
Gynecology and Obstetrics (FIGO) [10] have defined prolonged pregnancy as
42 completed weeks or more. It is a pregnancy lasting more than 2weeks
beyond the confirmed expected date of delivery (EDD). The expressions
postterm pregnancy, prolonged pregnancy, postdates pregnancy and
after-term pregnancy are used synonymously. They are therefore used
interchangeably in this review. However, they should not be confused with
the expressions 'hypermaturity' and  'postmaturity', which connote a
clinical condition of the newborn in addition to prolonged pregnancy
[1].


The accurate determination of the expected day of confinement (EDC) is a
key issue in the antenatal and neonatal periods for both clinical
obstetrics and research. It has profound personal, social and medical
implications for the expectant mother. There are problems in estimating
the incidence of prolonged pregnancy, not just because of differing
definitions but also because of incomplete recording of pregnancies,
differences between hospital and population surveys, differing policies
for induction of labor, and varying proportions of women with uncertain
dates  [11] . Between 4% and 14% (average 10%) of women are prepared to
reach 42 weeks gestation, and 2% to 7% (average 4%) to reach 43 weeks
gestation depending on the population studied [6]. There have been recent
suggestions that the duration of normal pregnancy may be related to
maternal characters, such as height [12] parity  [13] and race [14]. In
studies where conception has been estimated from basal body temperature
charts [15], and ultrasound measurements [16], it has been shown that the
error in menstrual dating is heavily skewed to the right - i.e., there is
a tendency to overestimate gestation.

In spite of all these evidences, the last menstrual period (LMP)
continues to be the basis for estimating the duration of pregnancy, on a
worldwide scale. Often this is unknown, in which case gestational age may
be estimated by ultrasonographic measurements of fetal parameters such as
the crown - rump length [17] until about 12 weeks and the biparietal
diameter (BPD) [18] from about 14 to 22 weeks' gestation. There is no
uniform dating policy when both a valid LMP and ultrsonographic dates are
available. In practice, many obstetrics and ultrasound departments follow
a 7-, 10-, or 14-day rule [19] whereby preference is given to menstrual
dates if they are within 7,10, or 14 days, respectively, from the
ultrsonographic estimate. However the random error in dating by
ultrasound measurement of the biparietal diameter (BPD) in the second
trimester has been estimated at 3.2 days  [20] and unlike menstrual dates
this error is normally distributed [21]. It is now clear that even if
menstrual dates are considered "certain" there is no advantage taking
them into consideration for calculating the expected date of delivery if
a dating ultrsonography result is available. Dating by ultrasonographic
biometry in the first half of pregnancy results in a more accurate
prediction of the delivery date than using menstrual date alone or in
combination with ultrasonography  [22]. The corollary to the above is
that the current definitions of preterm and postterm pregnancies may need
to be revised to allow the increased precision achieved by ultrasound.
Inclusion of menstrual data for determination of gestational age may lead
to incorrect clinical decisions  [21] .


DIAGNOSIS

It is obvious from the available literature that the correct diagnosis of
postdate pregnancy is very difficult. The World Health Organization
definition [9] of term pregnancy as the interval from 259 to 294 days of
menstrual age was based on statistical data derived from menstrual dates.
It has been shown that even if the LMP is recalled with accuracy, it will
not be a reliable indicator of the actual date of conception. This is
because the onset of ovulation within the menstrual cycle is erratic and
may also vary from one cycle to the next [23]. Dating policies have
important clinical implications.

Because of unreliability of the menstrual dating [24] and the fact that
most obstetric units induce labor for "postmaturity" or "postterm" based
on the menstrual history , this method of historical diagnosis will
result in a high proportion of women having induction of labor
unnecessarily for postmaturity.  However ultrasonography in the first
half of pregnancy will reduce the percentage of pregnancies classified as
postterm by WHO definition (>42 weeks) from 11.5 to 3.5 (i.e., by 70%).

Mothers, midwives and physicians are often uncertain as to which date
should be used, and this may lead to considerable parental confusion, A
uniform dating policy would reduce much of the uncertainty in pregnancy
dating [23]. The available evidences are strongly in support that dating
by ultrasonography alone is the most accurate method for predicting
expected date of confinement. Confinement occurred on the day predicted
in 3.6% if EDC was based on the LMP and in 4.3% if it was based on the
scan. Delivery took place within 7 days of the EDC in 49.5% cases when
LMP alone was used and in 55.2% if ultrasonography alone was used. If
this margin of error was widened to 10 days the corresponding figures
were 64.1% and 70.3% [22].

FETAL SURVEILLANCE
We now have available to us many forms of testing to follow the well
being of the postterm fetus while still in utero. There is still
considerable uncertainty, however, as to how well these tests measure
fetal compromise and how effective treatment is when we use these tests
in our management. Whatever form of testing is chosen, it is important to
remember that the condition of the fetus can change quickly and thus,
monitoring should be at frequent intervals, and that none of the tests
are immune from false positives, false negatives [25]. Perinatal
mortality rates (excluding lethal congenital anomalies) have been
reported to be as low as 1.1-1.2 per 1000 with close surveillance [26].
Post-term perinatal deaths continue to occur, however, and some of these
deaths have occurred within 24 to 48 hours of normal fetal assessment
[25, 27]. Furthermore, despite current techniques of fetal surveillance,
there continues to be reports of higher risks of fetal and neonatal
morbidity and operative delivery with postterm compared to term pregnancy
[28] . The summary of the available tests is given below.

Fetal movements counting
Fetal movement counting has been a popular method of fetal surveillance
as it allows the mother to participate actively in the evaluation of her
baby's health [29]. Although one small controlled trial suggested that
this form of monitoring might be effective in decreasing the perinatal
mortality rate, another larger trial undertaken in many centers in Europe
and USA showed no beneficial effect [30]. Neither of these trials focused
on Postterm pregnancy. In the Canadian Multi center Postterm Pregnancy
Trial, women allocated to the expectant group were asked to count daily
until they count six movements, or for two hours (whichever took less
time). If after two hours the woman had felt less than six movements; she
was to contact her Obstetricians for further evaluation. Only 2.2%
(38/1707) of women reported decreased fetal movements. The two women in
the trial with stillbirths (excluding lethal anomalies) did not report
decreased fetal movement counts [25] .


Amniotic fluid volume
The volume of amniotic fluid has been estimated to decrease by 150-170ml
per week after 42 weeks of pregnancy [31]. More recent investigations
using real-time ultrasound confirm decreasing amniotic fluid as
pregnancies continue past 41 weeks [32, 33]. The reason for this decrease
is not completely understood but may be due to decrease fetal urine
production [34] . Amniotic fluid volume as assessed by ultrasound has
been evaluated in blind and unblinded studies in post term pregnancies
and there is now good evidence that postterm pregnancy with no or low
volumes of amniotic fluid are at higher risk of adverse perinatal outcome
than pregnancies with a normal amount of amniotic fluid [35]. Low volumes
of amniotic fluid, assessed by ultrasound, in posterm pregnancy have been
defined in various ways: when the largest pocket of amniotic fluid is
less than 3cm in depth [35], when the sum of the depth of the largest
pocket of amniotic fluid in each of four quadrants of the uterus is less
than 5cm (amniotic fluid index) [36] or when the product of the length x
depth of the largest pocket is less than 60cm [6]. This test is currently
considered one of the most sensitive for postterm fetal surveillance and
has been used as part of expectant management in five of the randomized
controlled trials of induction of labour compared to expectant management
in postterm pregnancy [26],[38],[39],[40].


Amnioscopy/amniocentesis for assessing the presence of meconium :
It is known that postterm pregnancies are frequently complicated by
meconium staining of the amniotic fluid than term pregnancy and that
meconium staining of the amniotic fluid is associated with higher risks
of adverse perinatal outcome [41]. It is not unreasonable to suggest
therefore, that amnioscopy or amniocentesis might be a good screening
test for fetal compromise in postterm pregnancies. In fact two randomized
controlled trials comparing induction of labor with expectant management
have used amnioscopy as part of the surveillance for expectant management
[42]. One trial which assessed the effectiveness of weekly amniocentesis
(and induction of labor if meconium were present) in postterm
pregnancies, did not find this better than a program of weekly
contraction tests [43] .

Contraction stress tests:
Some authors have reported on the use of contraction stress tests as a
method of fetal surveillance in postterm pregnancy [43] . Both nipple
stimulation and intravenous oxytocin have been used as mechanisms for
inducing contractions. The test results appear in some populations to
correlate with outcome and some Obstetricians prefer this test method of
fetal surveillance. The disadvantages are that it takes time to
administer and, in the case of oxytocin challenge test, requires an
intravenous infusion.

Non-stress test:
The non-stress test has been a popular method of fetal surveillance for
postterm pregnancies [44]. Abnormal test results (non-reactivity and
decelerations) have been associated with higher risks of adverse
perinatal outcome [45]. Although the test is easy to administer, and was
used in seven of eleven randomized controlled trials of induction of
labor in postterm pregnancy, there is evidence to suggest that it is not
an effective method of fetal surveillance. The overview of randomized
trials of non-stress testing in the Oxford Database of Perinatal Trials
indicates that this testing may result in a higher, rather than a lower,
risk of perinatal death  because of false reassurance [46] .

Biophysical profile:
The biophysical profile is one of the most popular methods of fetal
surveillance for postterm pregnancies  [28]. The profile is performed
using real-time ultrasound and consists of four distinct measures: fetal
breathing, fetal movement, fetal tone and amniotic fluid volume with or
without the non-stress test. Many of those who use this as the primary
method of fetal surveillance in postterm pregnancy consider the amniotic
fluid volume to be the most important measure of the biophysical profile
for assessing fetal well-being and, if this one aspect is abnormal,
consideration should be given to expediting delivery [27].

Doppler ultrasound
Doppler ultrasound of the fetal vessels is the most recent addition to
the armamentarium of fetal surveillance for postterm pregnancy. Some
studies suggest that it may help to identify those pregnancies at higher
risk of adverse outcome [47], whereas others have not found this form of
testing to be particularly helpful [48].

Hormonal tests
Hormonal tests, such as serum or urinary oestriols or serum human
placental lactogen, have been used to monitor postterm pregnancies [44].
These methods of surveillance are no longer very popular. This is perhaps
more because of the cost of these tests and the time required obtaining a
result, than false diagnosis.


MANAGEMENT
The management of postterm pregnancy that is otherwise uncomplicated is
controversial . Central to this controversy is whether the fetus is at
increasing risk of deterioration as the pregnancy advances. The
management options available for consideration for postterm pregnancy are
'Active management' - when pregnancy is terminated by induction of labor
after 41 weeks of gestation. Cervical ripening agents such as
prostaglandins [49] are used to prepare the cervix and, if necessary,
oxytocin and amniotomy are also used. The other popular option is the
'expectant management' in which the pregnancy is allowed to progress to
42 weeks and beyond. Labor is induced only if the cervix is well effaced
or dilated, or both, or if fetal compromise occurs. The fetal condition
is evaluated by various techniques.

Postterm pregnancy has historically been considered a risk factor for
adverse perinatal outcome. Before the introduction of fetal surveillance
techniques, prolonged pregnancy has been associated with a two fold to
ten fold increase in the incidence of fetal distress in labor  [50] .
Induction of labor emerged as a means of reducing perinatal risks in the
prolonged pregnancy.

The development and application of modern techniques of fetal assessment
have been associated with a reduction in perinatal risk in prolonged
pregnancies. In 13 studies between 1978 and 1987 in which antenatal fetal
surveillance was used for follow-up of postterm pregnancies, the risk of
perinatal mortality was similar to that of pregnancies delivered at term
[51]. Such report demonstrated that expectant management of postterm
pregnancy was an acceptable alternative to induction of labor. However,
several recent studies have suggested that in spite of modern monitoring
techniques, the postterm fetus remains at risk for certain perinatal
morbidities such as meconium aspiration, fetal distress in labor and
macrosomia with its attendant complications [44, 52]. These findings have
rekindled the controversy surrounding the optimal management of
prolonged pregnancy. In response, several prospective randomized trials
using contemporary management schemes have compared induction and
expectant management in  prolonged pregnancy [53][54] . These trials have
yielded conflicting results that have been attributed to differences
in-patients selection (ripe Vs unripe cervix at entry), methods of labor
induction (PGE2 with or without oxytocin, amniotomy or stripping of the
membranes), and techniques of antenatal fetal surveillance [51].

There have been doubts expressed about the value of induction of labor in
prolonged pregnancy, mainly that it may result in more operative
intervention without necessarily preventing fetal hypoxia and perinatal
death from asphyxia. Furthermore, there is a perception among
obstetricians that women do not want induction of labor, which may stem
from the outcry in the lay press in the 1970s against induction. Thus in
many maternity units induction rates have been falling. This controversy
has been partly resolved by the results of sixteen randomized trials, a
meta-analysis of which provides clear answers to many of the questions
concerning induction of  labor  [25].

In conclusion, the available data suggests that induction of labor should
be recommended to women with certain dates at 41-plus weeks gestation,
for it will reduce the likelihood of perinatal mortality and of cesarean
section for fetal distress.

SHOULD INDUCTION OF LABOUR BE ROUTINE?
IS IT COST EFFECTIVE?
From the foregoing, the following conclusions can be deduced. Firstly,
contrary to what many obstetricians believe induction of labor for
prolonged pregnancy does not increase the likelihood of cesarean section,
rather, it decreases it. Secondly, the risk of fetal distress from
uteroplacental insufficiency due to prolonged pregnancy can be reduced by
induction of labor, even to the point of preventing perinatal death from
asphyxia. The available evidence suggests that although cesarean sections
may be few with induction of labor at 40 weeks, this is offset by an
increase in instrumental vaginal delivery. There is little justification
for policy of routine induction of labor at such a relatively early
gestation even though perinatal mortality is lowest at 40 weeks
gestation.

Should one recommend induction of labor at 41-plus weeks gestation to all
women with certain dates? Despite the evidence from 11 randomized trials
some obstetricians may not be convinced. The perinatal mortality with
induction of labor at 41-plus weeks is 0.3 per 1000 (one death in 2905
cases) and in the control group is 2.5 per 1000 (seven deaths in 2822
cases). Thus, in order to prevent one perinatal death, one would have to
induce as many as 460 women at 41-plus week's gestation. The reduction in
cesarean section rates in a particular maternity unit is 20%, 460
inductions would result in 13 fewer cesarean sections, hardly a huge
saving! Thus a big effort by way of induction of labor has to be
undertaken in order to secure comparatively a modest gain.

The result of The Canadian Multicentre Postterm Pregnancy Trial   [26]
and of the meta-analysis  [25] , combined with the significant difference
in cost between the two strategies (active Vs expectant management)
unambiguously support the induction strategy as a "win-win" alternative
in the management of postterm pregnancies. That is, an induction
management policy produces better outcome at a lower cost  [55]. These
findings also support recent recommendations that induction of labor
should be offered to women with pregnancies of 41 or more weeks [56].
Research is needed to determine the most effective methods of induction
(e.g. medication, nipple stimulation, stripping or sweeping of the
membranes or mechanical methods).


WOMEN'S VIEWS ABOUT MANAGEMENT OPTIONS
The controversy surrounding the management of prolonged pregnancy has
reached a critical point in recent times. The available data points to
the fact that a huge effort by way of induction of labor only secures a
modest gain  [30] . What research is there on women's views however is
reassuring. Thus Cardozo (1986) [53] asked women after delivery to
clarify their satisfaction with their treatment allocation into three
categories. Dissatisfaction was not associated with treatment allocation,
but was strongly associated with operative delivery, regardless of
treatment allocation. The study by Roberts et al. 1991 [57] showed that
the majority of women  preferred
induction.

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