Postterm Pregnancy
- Author: Aaron B Caughey, MD, PhD, MPH; Chief Editor:
David Chelmow, MD
Overview
Postterm
pregnancy is defined as a pregnancy that extends to 42 0/7 weeks and beyond.[1]
The reported frequency of postterm pregnancy is approximately
3-12%.[1, 2] However, the actual biologic
variation is likely less since the most frequent cause of a postterm pregnancy
diagnosis is inaccurate dating.[3,
4, 5, 6] Risk factors for actual postterm pregnancy include
primiparity, prior postterm pregnancy, male gender of the fetus, and genetic
factors.[7, 8, 9, 2, 1]
Laursen
et al studied monozygotic and dizygotic twins and their subsequent development
of prolonged pregnancies. They found that maternal but not paternal genetic
factors influenced the rate of postterm pregnancies and accounted for the
etiology in as many as 30% of these pregnancies.[10]
A more recently described risk factor is obesity, which
appears to increase the risk of pregnancies progressing beyond 41 or 42 weeks
of gestation.[11, 12, 13]
Although
the last menstrual period (LMP) has been traditionally used to calculate the
estimated due date (EDD), many inaccuracies exist using this method in women
who have irregular cycles, have been on recent hormonal birth control, or who
have first trimester bleeding. In particular, women are more likely to be
oligo-ovulatory than polyovulatory, so cycles longer than 28 days are not
uncommonly seen.[4] If such a cycle is 35 days instead of
28 days, a second trimester ultrasound will not be powerful enough to redate
the pregnancy. Thus, not only the LMP date, but the regularity and length of
cycles must be taken into account when estimating gestational age.
Ultrasonographic
dating early in pregnancy can improve the reliability of the EDD; however, it
is necessary to understand the margin of error reported at various times during
each trimester. A calculated gestational age by composite biometry from a
sonogram must be considered an estimate and must take into account the range of
possibilities.
Estimation
range varies. For example, crown-rump length (CRL) is 3-5 days, ultrasonography
performed at 12-20 weeks of gestation is 7-10 days, at 20-30 weeks is 2 weeks,
and after 30 weeks is 3 weeks. Thus, a pregnancy that is 35 weeks by a 31-week
ultrasound could actually be anywhere from 32 weeks to 38 weeks (35 wk +/-3
wk). If the calculated ultrasonographic gestational age varies from the LMP
more than the respective range of error, it is used instead to establish the
final EDD. The importance of determining by what method a pregnancy is dated
cannot be overemphasized because this may have significant consequences if the
physician delivers a so-called term pregnancy that is not or observes a
so-called term pregnancy that is very postterm.
When
determining a management plan for an impending postterm pregnancy (>40 wk of
gestation but < 42 wk), the 3 options are (1) elective induction of labor,
(2) expectant management of the pregnancy, or (3) antenatal testing. Each of
these 3 options may be used at any particular time during this 2-week period.
Note
that if the pregnancy is at risk for an adverse outcome from an underlying
condition, either maternal or fetal, inducing labor may proceed without
documented lung maturity. Also, an elective induction of labor may proceed at
or after 39 weeks of gestation in the absence of documented lung maturity
provided that 36 weeks have elapsed since documentation of a positive human
chorionic gonadotropin (+hCG) test finding, 20 weeks of fetal heart tones have
been established by a fetoscope or 30 weeks by a Doppler examination, or 39
weeks' gestation have been established by a CRL or by an ultrasound performed
before 20 weeks of gestation consistent with dates by the patient's LMP.
Perinatal outcomes in postterm pregnancies
Recent
studies have shown that the risks to the fetus[14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26] and to the
mother[23, 27, 28, 29, 30, 31, 32, 33] of
continuing the pregnancy beyond the estimated date of delivery is greater than
originally appreciated.
Risks
have traditionally been underestimated for 2 reasons. First, earlier studies
were published before the routine use of obstetric ultrasonography and, as a
result, likely included many pregnancies that were not truly postterm. As noted
above, such a misclassification bias would artificially lower the complication
rates of pregnancies designated postterm and increase the complication rates in
those designated term, resulting in a diminution in the difference between term
and postterm pregnancies.
The
second issue relates to the definition of stillbirth rates. Traditionally,
stillbirth rates were calculated using all pregnancies delivered at a given gestational
age as the denominator. However, once a fetus is delivered, it is no longer at
risk of intrauterine fetal demise, and use of this denominator has
traditionally underestimated the risk of stillbirth. The appropriate
denominator is not all deliveries at a given gestational age, but ongoing
(undelivered) pregnancies.[18,
19, 33] In one retrospective study of more than 170,000
singleton births, for example, Hilder et al demonstrated that the stillbirth
rate increased 6-fold (from 0.35-2.12 per 1,000 pregnancies) when the
denominator was changed from all deliveries to ongoing (undelivered)
pregnancies.[16]
Fetal and neonatal risks
Antepartum
stillbirths account for more perinatal deaths than either complications of
prematurity or sudden infant death syndrome.[17]
Perinatal mortality (defined as stillbirths plus early
neonatal deaths) at 42 weeks of gestation is twice that at 40 weeks (4-7 vs 2-3
per 1,000 deliveries, respectively) and increases 4-fold at 43 weeks and 5- to
7-fold at 44 weeks.[15, 16, 17] These data also demonstrate
that, when calculated per 1000 ongoing pregnancies, fetal and neonatal
mortality rates increase sharply after 40 weeks.[16]
Cotzias
et al calculated the risk of stillbirth in ongoing pregnancies for each
gestational age from 35-43 weeks.[17]
The risk of stillbirth was 1 in 926 ongoing pregnancies at 40
weeks’ gestation, 1 in 826 at 41 weeks, 1 in 769 at 42 weeks, and 1 in 633 at
43 weeks. Uteroplacental insufficiency, asphyxia (with and without meconium),
intrauterine infection, and anencephaly all contribute to excess perinatal
deaths, although postterm anencephaly is essentially nonexistent with modern
obstetrical care.[34]
A
number of key morbidities are greater in infants born to postterm pregnancies
as well as pregnancies that progress to and beyond 41 0/7 weeks gestation
including meconium and meconium aspiration, neonatal acidemia, low Apgar
scores, macrosomia, and, in turn, birth injury. For example, since postterm
infants are larger than term infants, with a higher incidence of fetal
macrosomia (defined as estimated fetal weight ≥ 4,500 g)[35]
, they are, in turn, at greater risk for other complications.[36,
37] Such complications associated with fetal macrosomia
include prolonged labor, cephalopelvic disproportion, and shoulder dystocia
with resultant risks of orthopedic or neurologic injury.
Approximately
20% of postterm fetuses have fetal dysmaturity (postmaturity) syndrome, which
describes infants with characteristics of chronic intrauterine growth
restriction from uteroplacental insufficiency.[38]
These pregnancies are at increased risk of umbilical cord
compression from oligohydramnios, nonreassuring fetal antepartum or intrapartum
assessment, intrauterine passage of meconium, and short-term neonatal complications
(such as hypoglycemia, seizures, and respiratory insufficiency).
Meconium
aspiration syndrome refers to respiratory compromise with tachypnea, cyanosis,
and reduced pulmonary compliance in newborns exposed to meconium in utero and
is seen in higher rates in postterm neonates.[39]
Indeed, the 4-fold decrease in the incidence of the meconium
aspiration syndrome in the United States from 1990-1998 has been attributed
primarily to a reduction in the postterm delivery rate[21]
with very little contribution from conventional interventions
designed to protect the lungs from the chemical pneumonitis caused by chronic
meconium exposure, such as amnioinfusion[40,
41] or routine nasopharyngeal suctioning of meconium-stained
neonates.[42]
Postterm
pregnancy is also an independent risk factor for neonatal encephalopathy[43]
and for death in the first year of life.[16,
17]
While
much of the work above has been conducted in postterm pregnancies. Some of the
fetal risks such as presence of meconium, increased risk of neonatal acidemia,
and even stillbirth have been described as being greater at 41 weeks of
gestation and even at 40 weeks of gestation as compared with 39 weeks’
gestation.[22, 23] For example, in one study, the
rates of meconium and neonatal acidemia both increased throughout term
pregnancies beyond 38 weeks of gestation. In addition to stillbirth being
increased prior to 42 weeks of gestation, one study found that the risk of
neonatal mortality also increases beyond 41 weeks of gestation.[44]
Thus, 42 weeks does not represent a threshold below which risk
is uniformly distributed. Indeed, neonatal morbidity (including meconium
aspiration syndrome, birth injury, and neonatal acidemia) appears to be the
lowest at around 38 weeks and increase in a continuous fashion thereafter.[45]
While
preterm delivery is a well-established risk factor for cerebral palsy, a recent
study suggested that delivery at 42 weeks or later is also associated with
increased risk (RR 1.4, 95% CI, 1.2-1.6 when compared with delivery at 40
weeks’ gestation).[46]
Maternal risks and mode of delivery
The
maternal risks of postterm pregnancy are often underappreciated. These include
an increase in labor dystocia (9-12% vs
2-7% at term), an increase in severe perineal injury (3rd and 4th
degree perineal lacerations) related to macrosomia (3.3% vs 2.6% at term) and
operative vaginal delivery, and a doubling in the rate of cesarean delivery
(14% vs 7% at term).[18, 27, 28, 29] The latter is associated
with higher risks of complications such as endometritis, hemorrhage, and
thromboembolic disease.[28,
47]
In
addition to the medical risks, the emotional impact (anxiety and frustration)
of carrying a pregnancy 1-2 weeks beyond the estimated due date should not be
underestimated. In a randomized, controlled trial of women at 41 weeks of
gestation, women who were induced would desire the same management 74% of the
time, whereas women with serial antenatal monitoring only desired the same
management 38% of the time.[48]
Similar
to neonatal outcomes, maternal morbidity also increases in term pregnancies
prior to 42 weeks of gestation. Such complications as chorioamnionitis, severe
perineal lacerations, cesarean delivery rates, postpartum hemorrhage, and
endomyometritis all increase progressively after 39 weeks of gestation.[23,
30, 31, 32, 21]
Timing of Delivery
The
first decision that must be made when managing an impending postterm pregnancy
is whether to deliver. In certain cases (eg, nonreassuring surveillance,
oligohydramnios, growth restriction, certain maternal diseases), the decision
is straightforward. In these high-risk situations, the time at which the risks
of remaining pregnant begin to outweigh the risks of delivery may come at an
earlier gestational age (eg, 39 weeks of gestation). However, frequently
several options can be considered when determining a course of action in the
low-risk pregnancy. The certainty of gestational age, cervical examination
findings, estimated fetal weight, patient preference, and past obstetric
history must all be considered when mapping a course of action.
The
main argument against a policy of routine induction of labor at 41 0/7 to 41
6/7 weeks has been that induction increases the rate of cesarean delivery
without decreasing maternal and/or neonatal morbidity. Some of the studies that
failed to show a reduction in fetal/neonatal morbidity were diluted by poorly
dated pregnancies that were not necessarily postterm. In addition, the
potential for increasing the risk for cesarean delivery with a failed induction
is far less likely in the era of safe and effective cervical ripening agents.
To
date, more than 10 studies have been published of elective induction of labor,
many of them at 41 weeks of gestation.[49,
34, 50, 51, 52, 53] The preponderance of the evidence from
these studies, including meta-analyses, find that not only is rate of cesarean
delivery not increased in women who were randomized to routine induction of
labor, but also more cesarean deliveries were performed in the noninduction
groups, and the most frequent indication was fetal distress. Even with multiple
studies, very few neonatal differences have been demonstrated. However, the
reduction in meconium is statistically significant and the rate of neonatal
mortality is lower.
In
summary, routine induction at 41 weeks of gestation does not increase the
cesarean delivery rate and may decrease it without negatively affecting
perinatal morbidity or mortality. In fact, both the woman and the neonate
benefit from a policy of routine induction of labor in well-dated, low-risk
pregnancies at 41 weeks' gestation. Because it is associated with a lower rate
of adverse outcomes, including shoulder dystocia and meconium aspiration
syndrome, this policy may also prove to be more cost-effective.[54]
A
policy of routine induction at 40 weeks' has few benefits, and there are
multiple reasons not to allow a pregnancy to progress beyond 42 weeks.
Prior
to 41 weeks of gestation, the evidence becomes more scant with only 3 small,
non-US, randomized, controlled trials comparing elective induction of labor to
expectant management of pregnancy.[52]
However, elective induction of labor is increasingly being
used as a management strategy.[55,
56] While this management may be reasonable in a practice that
allows 48 hours or more for the management of the latent phase and the first
stage of labor overall, in a setting where induction of labor is called a
failure after 18-24 hours, it will likely further increase the cesarean
delivery rate
Prevention of Postterm Pregnancy
As
noted above, the most decisive way to prevent postterm pregnancy is induction
of labor prior to 42 weeks’ gestation. However, since complications rise during
40 and 41 weeks' gestation and both clinicians and patients are concerned about
the risks of induction of labor, it is perceivably better for women to go into
spontaneous labor at 39 weeks of gestation on their own. Several minimally
invasive interventions have been recommended to encourage the onset of labor at
term and prevent postterm pregnancy, including membrane stripping, unprotected
coitus, and acupuncture.
Stripping
or sweeping of the fetal membranes refers to digital separation of the
membranes from the wall of the cervix and lower uterine segment. This
technique, which likely acts by releasing endogenous prostaglandins from the
cervix, requires the cervix to be sufficiently dilated to admit the
practitioner’s finger. Although stripping of the membranes may be able to
reduce the interval to spontaneous onset of labor, a reduction in operative
vaginal delivery, cesarean delivery rates, or maternal or neonatal morbidity
has not been consistently proven.[57,
58, 59]
Unprotected
sexual intercourse causes uterine contractions through the action of
prostaglandins in semen and potentially release of endogenous prostaglandins
similar to stripping of the membranes. Indeed, prostaglandins were originally
isolated from extract of prostate and seminal vesicle glands, hence their name.
Despite some conflicting data, it appears that unprotected coitus may lead to
the earlier onset of labor, reduction in postterm pregnancy rates, and less
induction of labor.[60, 61, 62]
In
a small randomized trial that attempted to address this question, women were
randomized to a group advised to have coitus versus a control group that was
not. In this study, the women advised to have coitus did so more often (60% vs
40%), the difference in the rate of spontaneous labor was not measurable in
this underpowered study.[63]
Similarly, the efficacy of acupuncture for induction of labor
cannot be definitively assessed because of the paucity of trial data; this
requires further examination.[64,
65]
Cervical Ripening and Intrapartum Management
Once
the decision to deliver a patient has been made, the management of the labor
induction depends on the clinical setting, and a brief review of cervical
ripening agents and potential complications of induction of labor is
appropriate. A comprehensive review of all available methods for cervical
ripening, indications, contraindications, and dosing is beyond the scope of
this article.
As
many as 80% of patients who reach 42 weeks' gestation have an unfavorable
cervical examination (ie, Bishop Score < 7). Many options are available for
cervical ripening. The different preparations, indications, contraindications,
and multiple dosing regimes of each require practitioners to familiarize
themselves with several of the preparations.
Prostaglandin
E2 gel and suppositories for vaginal application were used extensively until
the late 1990s when many pharmacies stopped manufacturing them because of the
advent of commercially available and less labor-intensive preparations.
Currently available chemical preparations include prostaglandin E1 tablets for
oral or vaginal use (misoprostol), prostaglandin E2 gel for intracervical
application (dinoprostone cervical [Prepidil]), and a prostaglandin E2 vaginal
insert (dinoprostone [Cervidil]). Cervidil contains 10 mg of dinoprostone and
has a lower constant release of medication than Prepidil. In addition, this
vaginal insert device allows for easier removal in the event of uterine
hyperstimulation.
Many
studies have compared the efficacy and risks of various prostaglandin cervical
ripening agents. Rozenburg et al performed a randomized trial comparing intravaginal
misoprostol and dinoprostone vaginal insert in pregnancies at high risk of
fetal distress. They found that both methods were equally safe for the
induction of labor and misoprostol was actually more effective.[66]
Another
method for ripening the cervix is by mechanical dilation. These devices may act
by a combination of mechanical forces and by causing release of endogenous
prostaglandins. Foley balloon catheters placed in the cervix, extra-amniotic
saline infusions, and laminaria have all been studied and have been shown to be
effective.
Regardless
of what method is chosen for cervical ripening, the practitioner must be aware
of the potential hazards surrounding the use of these agents in the patient
with a scarred uterus. In addition, the potential for uterine tachysystole and
subsequent fetal distress requires that care be taken to avoid using too high a
dose or too short a dosing interval in an attempt to get a patient delivered
rapidly. Care should also be taken when using combinations of mechanical and
pharmacologic methods of cervical ripening.
Once
an induction of labor has begun, watch for the major potential complications
associated with inductions beyond 41 weeks' gestation and have a plan for
dealing with each. Complications include the presence of meconium, macrosomia,
and fetal intolerance to labor.
The
further the pregnancy progresses beyond 40 weeks, the more likely it is that
significant amounts of meconium will be present. This is due to increased
uteroplacental insufficiency, which leads to hypoxia in labor and activation of
the vagal system. In addition, the presence of a smaller amount of amniotic
fluid increases the relative concentration of meconium in utero.
Traditionally,
saline amnioinfusion and aggressive nasopharyngeal and oropharyngeal suctioning
at the perineum were used to decrease the risk of meconium aspiration syndrome.
Recent studies contradict this standard practice. Fraser et al performed a prospective,
randomized, multicenter study evaluating the risks and benefits of
amnioinfusion for the prevention of meconium aspiration syndrome.[41]
They concluded that in clinical settings, which have
peripartum surveillance, amnioinfusion of thick meconium-stained amniotic fluid
did not decrease the risk of moderate-to-severe meconium aspiration syndrome,
perinatal death, or other serious neonatal disorders compared with expectant
management. In addition, other recent studies have shown that deep suctioning
of the airway at the perineum does not effectively prevent meconium aspiration
syndrome, contrary to popular belief.
Fetal
macrosomia can lead to maternal and fetal birth trauma and to arrest of both
first- and second-stage labor. Because the risk of macrosomia increases
throughout term and postterm pregnancies, one of the most important parts of
the delivery plan is being prepared for shoulder dystocia in the event that
this unpredictable, anxiety-provoking, and potentially dangerous condition
arises. To prepare such an event, experienced clinicians should be present at
the delivery, a stool/step next to the delivery bed should be placed to help
with suprapubic pressure, and the maneuvers to reduce the shoulder dystocia
should be reviewed.
Finally,
intrapartum fetal surveillance in an attempt to document fetal intolerance to
labor before it leads to acidosis is critical. Whether continuous fetal
monitoring or intermittent auscultation is used, interpretation of the results
by a well-trained clinician is of paramount importance. If the fetal heart rate
tracing is equivocal, fetal scalp stimulation and/or fetal scalp blood sampling
may provide the reassurance necessary to justify continuing the induction of
labor. If the practitioner cannot find reassurance that the fetus is tolerating
labor, cesarean delivery is recommended.
Summary
The
management of postterm pregnancies is complicated and fraught with complex
issues. The decision of whether to induce labor or to proceed with expectant
management with or without antepartum fetal surveillance is not taken lightly.
Data support inducing labor at 41 weeks' gestation in an accurately dated,
low-risk pregnancy, regardless of cervical examination findings. This strategy,
although not without its critics, averts the need for antepartum fetal
surveillance and does not increase the cesarean delivery rate; in fact, it may
decrease the cesarean delivery rate.
