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Chapter 25. Postterm pregnancy
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CHAPTER 25. POSTTERM PREGNANCY
A postterm pregnancy is one that has extended beyond 42 weeks or 294 days from the first day of the last menstrual period.
The mean duration of a normal pregnancy, calculated from the 1st day of the last menstrual period, is 280 days or 40 + 0 weeks of gestation (GW).
A pregnancy prolonged by more than 14 days, in other words, a pregnancy lasting 294 days or 42 + 0 GW or more, is defined by the WHO and FIGO as prolonged, post-term or post-mature pregnancy.
Of note:
· early term is defined as 37+0 to 38+6 weeks of gestation,
· full term is defined as 39+0 to 40+6 weeks of gestation,
· and late term is 41+0 to 41+6 weeks of gestation.
Incidence.
The reported frequency of postterm pregnancy is approximately 3-12%.
Consequences of postterm pregnancy
The risks to the fetus and to the mother of continuing the pregnancy beyond the estimated date of delivery is greater than originally appreciated.
Risks to the fetus
Perinatal mortality
Postterm pregnancy is associated with an increased risk of 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.
Perinatal morbidity
The risk and frequency of fetus damage is increased in postterm pregnancy because of fetal dysmaturity (postmaturity) syndrome. This refers to a fetus whose growth in the uterus after the due date has been restricted, usually due to a problem with delivery of blood to the fetus through the placenta (known as “aging of the placenta”). It is manifested by signs of chronic intrauterine growth retardation (IUFGR), and intrauterine fetal distress. More importantly: against the background of IUGR the postterm fetus has a highly developed central nervous system (CNS), which, of course, requires more oxygen. This causes a greater sensitivity of the fetus to a lack of oxygen, thus, even normal, adequate, uterine contractions in labor lead to acute fetal asphyxia up to intranatal fetal death. The situation is ambiguous and critical, and becomes more marked with more prolonged pregnancy Neonatal morbidityA number of key morbidities are greater in infants born to postterm pregnancies: meconium aspiration, neonatal acidemia, low Apgar scores, macrosomia, neonatal hypotrophy, neonatal encephalopathy, etc. Neonatal encephalopathy is a risk factor for death in the first year of life.
Large body size — Postterm fetuses may have a greater chance of developing complications related to larger body size (called macrosomia), which is defined as weighing more than 4500 grams. Complications can include prolonged labor, cephalopelvic disproportion and shoulder dystocia with resultant risks of birth traumatism (eg, fractured bones or nerve injury). Postterm pregnancy is also a risk factor for neonatal encephalopathy and for death in the first year of life.
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Risks to the mother
The maternal risks of postterm pregnancy are also significant. 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) of cesarean birth with its associated risks of bleeding, infection, injury to surrounding organs.
Etiology.
In most cases, the cause of postterm pregnancy is unknown. The incidence is higher in first pregnancies (elderly primigravidae or elderly multiparae are more likely to have postterm pregnancy), and in women who have had a previous postterm pregnancy. Genetic factors may also play a role. One study showed an increased risk of postterm pregnancy in women who were, themselves, born postterm. Other factor that may contribute to postterm pregnancy include: maternal obesity, sulfatase deficiency in the placenta, central nervous system abnormalities, fetal anencephaly, and male sex, the disturbances of menstrual cycle which can lead to hormonal failure, previous threatened abortions and their intensive therapy, nervous diseases, stresses.
Diagnosis is based on the anamnesis, external and internal obstetric examination, ultrasound examination. Biophysical profile, cardiotocography, colpocytology of the vaginal smears can help in differentiating true postmaturity from simple prolonged pregnancy.
Clinical signs of postmaturity include:
· weight loss — a regular periodic weight checking reveals a stationary or decreasing weight; the latter is due to diminishing liquid;
· decrease of the abdomen circumference — normally, the girth of the abdomen at the level of umbilicus increases steadily up to the completion of 38 weeks and then remains constant up to terms. Thereafter, the girth gradually diminishes because of diminishing liquid;
· reduction in uterine size and decrease in fetal motion.
· hardening of the skull bones revealed at vaginal examination or ultrasonic scanning.
Sonography helps to confirm the fetal maturity and detect evidences of placental insufficiency. Placental insufficiency due to “ageing” is one of the most typical features of the true post-term pregnancy. And it is the main reason of the fetal pathology (hypoxia, hypotrophy). Cephalometry and measurement of biparietal diameter equal to 9. 8 cm signifies maturity and, if it exceeds 10. 1 cm, the diagnosis of post-term pregnancy is certain. Reduction of amniotic fluid volume to less than 250 ml is suggestive of not only postmaturity but also a significant indicator of placental insufficiency. Placenta is reduced in thickness, while the body length increases, the supposed body weight decreases that testify to fetal hypotrophy because of placental insufficiency.
· Antenatal cardiotocography will find out deceleration patterns of fetal heart rate with uterine contractions, and it is the earliest evidence of fetal compromise.
· Amniocentesis and biochemical investigations of amniotic fluid
· A lecithin/sphingomyelin ratio greater 2, a positive “shake test” testify to maturity of the fetus. Postmaturity can be confirmed by finding yellow fluid during amniocentesis (or amnioscopy), caused by meconium staining of the amniotic fluid.
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· Cytologic examination — the presence of orange cells (over 50%) when stained with 0. 1% Nile blue sulphate is suggestive of postmaturity.
· Evaluation of urinary or serum estriol shows a stable or falling level.
Clinical significance. Post-term pregnancy and labors are dangerous for the mother and for the baby. During pregnancy the hypoxic state is aggravated because of placental “ageing”, marked placental insufficiency. During labor there is an increased incidence of asphyxia and intracranial damage due to:
· aggravating of pre-existing hypoxia, development of fetal distress;
· meconium aspiration syndrome and atelectasis resulting from premature occurrence of respiratory efforts owing to intrauterine anoxia with consequent inhalation of meconium containing liquor amnii;
· increased incidence of complicated labor owing to big size of the fetal head, non-moulding of the head due to hardening of the skull bones and occasional shoulder dystocia. So the incidence of craniopelvic disproportion increases, which leads to a high rate of abdominal delivery;
· abnormalities of labor pains, more often weakness of labor pains.
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