Post-Date and post-term
Pregnancies
Defenition
*Postterm pregnancy includes pregnancies that last
longer than 42 weeks.
*Postdate pregnancies last longer than the established
or estimated date of
confinement (ie, 40 wk).
Incidence
The reported frequency
of postterm pregnancy is approximately 3-12%.
Assuming that the
incidence of postdate pregnancy may be slightly higher
than that of postterm
pregnancy is reasonable.
Aetiology
Unknown, but hereditary, hormonal and
non-engagement of the presenting part are suspected
factors.
An obvious cause of
both postterm and postdate pregnancy is inaccurate
dating criteria.
Ultrasound dating can
be inaccurate for a patient who presents late in
pregnancy and is unsure of her last menstrual period (LMP).
N.B
When using ultrasound
criteria for the dating of a pregnancy, it is necessary
to understand the margin of error reported at various
times during each trimester.
A calculated
gestational age by composite biometry from an ultrasound
must be considered an estimate and must take into
account the range of possibilities.
e.g
*crown-rump
length (CRL) is 3-5 days,
* ultrasound
performed at 12-20 weeks of gestation is 1 week,
*at 20-30
weeks is 2 weeks, and
*after 30
weeks is 3 weeks. Thus, a pregnancy that is 35 weeks by
ultrasound could actually be anywhere from 32 weeks to
38 weeks (35 wk +/- 3 wk).
Transcerebellar Diameter
A way to more
accurately date a pregnancy when composite biometry is
not consistent in all of the parameters (ie, biparietal
diameter [BPD], head circumference, abdominal
circumference, femur length), is to evaluate the
transcerebellar diameter. The diameter in millimeters
corresponds to weeks of gestation up to 24 weeks;
thereafter, charts are available to calculate
gestational age.
If the
transcerebellar diameter cannot be obtained, the
criterion
standard for dating in the second trimester is the BPD.
Risk of Post-term
1.
Placental insufficiency :
which may lead to growth restriction, fetal hypoxia or
even death.
2.
Oligohydramnios: with its
sequelae particularly cord compression during labor.
3.
Obstructed labor: due to
- Oversized
baby,. The risk for macrosomia, shoulder dystocia and
cephalopelvic disproportion increase in postterm
pregnancy
-
no moulding of the skull due to more calcification.
4- Increased incidence
of operative delivery.
5-The risk for perinatal
mortality increases in the postterm pregnancy
NB. Perinatal morbidity and mortality do
not increase appreciably between 40-41 weeks of
gestation; however, several complications are associated
with longer gestations
Diagnosis
(A) Antenatal:
-
History : calculation
of gestational age (see later).
-
Examination : larger
baby size.
-
X-ray : large
ossification center in the upper end of the tibia.
-
Ultrasonography: can
detect,
(B) Postnatal:
-
Baby length: more
than 54 cm.
-
Baby weight: more
than 4.5 kg.
-
Skull : well ossified
with smaller fontanelles.
-
Finger nails: project
beyond finger tips.
Management
To deliver or not:
W hen
determining a management plan for postdate pregnancy
(>40 wk of gestation but <42 wk),
*the first
decision is whether to deliver a patient and, if so,
* when and
by what route.
high-risk
situation
The first decision
that must be made when managing a postdate 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.
NB If 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:
1-36 weeks
have elapsed since documentation of a positive human
chorionic gonadotropin (+hCG) test finding,
2- 20 weeks
of fetal heart tones have been established by a
fetoscope or
3- 30 weeks
by a Doppler examination, or
4- 39 weeks’
gestation have been established by a CRL or
5- by an ultrasound
performed before 20 weeks of gestation consistent with
dates by the patient's Last Menstrual Period.
Low-risk
situation
There are
frequently several options to consider when determining
a course of action in the low-risk pregnancy.
1-The
certainty of gestational age,
2-cervical
examination findings,
3-estimated
fetal weight,
4-and past
obstetrical history
These
factors must all be considered when mapping a course of
action.
Involving the patient
in this discussion is wise because her feelings and
understanding of the situation are important as well.
NB
*pregnancy
should not be allowed to progress beyond 42 weeks of
gestation.
*The
question of how a pregnancy between 41-42 weeks should
be managed remains.
-the main argument
against a policy of routine induction of labor at 41-42
weeks is that induction increases the rate of cesarean
delivery without decreasing maternal and/ or neonatal
morbidity
- A recent
review in the Cochrane Library concluded that routine
induction in low-risk pregnancies at or after 41 weeks’
gestation is associated with a reduction in perinatal
mortality, with no increase in the rate of instrument
deliveries or cesarean delivery.
Expectant Management
If the physician
decides not to deliver.... the decision whether to
institute antepartum fetal surveillance and what
method(s) of surveillance to use must be discussed with
the patient.
Antepartum fetal
surveillance should be used in postterm pregnancies (ie,
>42 wk of gestation) when delivery is not performed
(usually for obstetrical contraindications).
Evidence to
suggest that antepartum surveillance improves outcomes
before 41 weeks’ gestation in low-risk pregnancies is
insufficient, and routine use of antepartum surveillance
between 40-41 weeks’ gestation is not supported by the
literature.
* An
amniotic fluid index of more than 8 cm and a reactive
fetal heart rate tracing are reassuring.
-When performing the
fetal heart rate tracing, the patient may be seated or
in a lateral recumbent position with lateral tilt.
-Two accelerations in
the fetal heart rate of 15 beats per minute over
baseline in a 20-minute period constitute a reactive
tracing.
-If the
tracing is not reactive within the first 20 minutes, the
test may be extended another 20 minutes.
-If the
tracing remains nonreactive, a backup test must be
performed in order to be sure that the intrauterine
environment is still safe.
*Back-up
tests:
-A
contraction stress test or
- A full
biophysical profile
are both
acceptable backup tests.
These may
also be used if the tracing is reactive but shows fetal
heart rate decelerations.
However, in
the pregnancy that is beyond 41 weeks of gestation, the
threshold for delivery should be very low.
NB
*Twice-weekly testing of patients at risk for fetal
distress was superior to weekly testing,
* A
modified biophysical profile consisting of a nonstress
test and an amniotic fluid index have been shown to be
as sensitive as a full biophysical profile.
Induction of Labor and
Intrapartum Management
When the
decision to deliver a patient has been made, the route
of delivery and the specifics of intrapartum management
depend on individual circumstances.
About 80%
of patients who reach 42 weeks’ gestation have an
unfavorable cervical examination finding (ie, Bishop
score <7)
Cervical Ripening
Chemical
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, prostaglandin E2 gel
for intracervical application, and a vaginal insert
containing 10 mg of dinoprostone.
The
progesterone antagonist RU486 may become available, but
it has been studied less than other pharmacologic
agents.
Mechanical
These
devices may act by a combination of mechanical forces
and by causing release of endogenous prostaglandins.
*Membrane
sweeping or stripping
* Foley
balloon catheters placed in the cervix
*
Extra-amniotic saline infusions, and laminaria
All have all
been studied and have been shown to be effective .
NB- Be
aware of the potential hazards surrounding the use of
these
agents in the patient with a scarred uterus.
-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.
-It is necessary
to watch for the major potential complications
associated with inductions beyond 41 weeks’
gestation and to have a plan for dealing with each.
Complications include
*the
presence of meconium,
*macrosomia, and
*fetal
intolerance to labor.
Meconium
The farther
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
-the
presence of less amniotic fluid increases the relative
amount of meconium in utero.
To counter the effects
of meconium, a combination of amnioinfusion of isotonic
sodium chloride solution and prompt suctioning of the
oropharynx and nose upon delivery of the head must be
considered.
Macrosomia
Fetal
macrosomia can lead to maternal and fetal birth trauma
and to arrest of both first- and second-stage labor.
-Estimated
fetal weight should be documented prior to beginning a
postdate induction.
-Mid-pelvic
instrument deliveries should not be attempted.
-The most
important part of a delivery plan is being prepared for
shoulder dystocia
in the
event that this unpredictable, anxiety-provoking, and
potentially
dangerous condition arises.
Fetal
Intolerance to labor
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, fetal scalp blood sampling, and/or fetal
pulse oximetry may provide the reassurance necessary to
justify continuing the induction of labor.
If no
reassurance that the fetus is tolerating labor,
cesarean delivery is recommended.
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