Pregnancy reaching or exceeding 42 weeks of gestation (≥294 days) without delivery is classified as postterm pregnancy. In recent years, active management of pregnancies at or beyond 41 weeks has significantly reduced the incidence of postterm pregnancy.
Pathophysiology
Placenta
Postterm pregnancy is associated with two types of placental changes:
- Some placentas maintain normal function, showing an appearance and microscopic findings similar to those of term placentas, except for slightly increased weight.
- Other placentas show dysfunction, with significant reductions in processes such as material exchange, defense mechanisms, synthesis, and immune functions.
Amniotic Fluid
After 38 weeks of gestation, the volume of amniotic fluid gradually decreases as gestation progresses. By 42 weeks, amniotic fluid levels often decrease rapidly, with approximately 30% of cases showing levels reduced to less than 300 mL. The presence of meconium-stained amniotic fluid increases significantly, occurring at 2–3 times the rate observed in term pregnancies. When oligohydramnios coexists, the rate of meconium-stained amniotic fluid reaches 71%.
Fetus
Fetal growth patterns in postterm pregnancy depend on placental function and may be categorized into the following types:
Normal Growth or Macrosomia
When placental function remains normal, the fetus may continue to grow, with approximately 25% of cases resulting in macrosomia. Among these, 5.4% of fetuses have a birth weight exceeding 4,500 g.
Postmaturity Syndrome
Fetuses with postmaturity syndrome exhibit characteristic appearances related to placental dysfunction, poor placental perfusion, fetal hypoxia, and nutritional deficiencies. Typical features include dry, loose, wrinkled, and peeling skin, especially on the palms and soles; a slender body with diminished subcutaneous fat and absence of vernix caseosa, suggesting wasting; thick hair and long fingernails (or toenails); and an appearance resembling a "small old person," with alertness and signs of distress. Yellow staining of the skin, amniotic sac, and umbilical cord is common due to reduced amniotic fluid levels and the presence of meconium.
Fetal Growth Restriction (FGR):
Growth restriction may coexist with postterm pregnancy, further increasing fetal risks. Approximately one-third of stillbirths in postterm pregnancies occur in growth-restricted fetuses.
Maternal and Perinatal Impacts
Perinatal Impacts
In addition to the manifestations of postmaturity syndrome, the incidence and mortality rates of conditions such as fetal distress, stillbirth, meconium aspiration syndrome, neonatal asphyxia, and macrosomia increase significantly.
Maternal Impacts
Postterm pregnancy is associated with prolonged labor and higher rates of labor dystocia, leading to increased rates of operative delivery and maternal birth trauma.
Diagnosis and Assessment of Fetal Condition
Accurate gestational age verification is essential for the diagnosis of postterm pregnancy.
Verifying Gestational Age
History
For individuals with regular menstrual cycles (28–30 days), postterm pregnancy is diagnosed if no delivery occurs by 42 weeks of gestation, calculated from the first day of the last menstrual period. For individuals with cycles longer than 30 days, this timeframe may be extended accordingly.
For individuals with irregular menstrual cycles, conception during lactational amenorrhea, or unclear recall of the last menstrual period, estimated delivery dates can be calculated based on basal body temperature–based ovulation data. If no delivery occurs 280 days after ovulation, postterm pregnancy is diagnosed.
Estimated delivery dates can also be calculated based on the timing of sexual intercourse.
For pregnancies achieved through assisted reproductive techniques (e.g., artificial insemination or in vitro fertilization–embryo transfer), estimated delivery dates are calculated based on procedural dates.
Clinical Features
Factors such as the timing of early pregnancy symptoms, the onset of fetal movements, and uterine size during early pregnancy assessments provide insights into gestational age estimation, though their precision is limited.
Auxiliary Tests
Gestational age can be determined based on ultrasound examination. Ultrasonography within the first 22 weeks of gestation is the most reliable method, with the crown-rump length (CRL) providing the most accurate estimates during early pregnancy. In the second trimester, the combination of biparietal diameter, abdominal circumference, and femur length aids in estimating both gestational age and fetal growth and development.
Early pregnancy blood or urine human chorionic gonadotropin (hCG) levels may provide additional gestational age estimates.
Assessing Fetal Condition
Fetal Movement
Reduced fetal movement detected through self-monitoring may indicate intrauterine fetal hypoxia.
Electronic Fetal Heart Rate Monitoring
A nonreactive result on a non-stress test (NST) requires further evaluation with a contraction stress test (CST). Recurrent late decelerations suggest placental dysfunction and significant fetal hypoxia, while variable decelerations often indicate umbilical cord compression, frequently associated with oligohydramnios.
Ultrasound
Biophysical profile scoring evaluates parameters such as fetal movement, muscle tone, breathing movements, and amniotic fluid volume. Additionally, Doppler measurement of umbilical artery blood flow provides insights into the fetal intrauterine condition.
Management
The incidence of perinatal complications and associated mortality rates increases after 41 weeks of gestation. Therefore, when pregnancy reaches or exceeds 41 weeks, termination of pregnancy should be considered to avoid postterm pregnancy. Once diagnosed as postterm pregnancy, termination of pregnancy is indicated. The method of termination should be determined based on a comprehensive analysis of factors including the fetal condition, fetal size, cervical ripeness, and prenatal monitoring results, in order to choose the most appropriate approach.
Cervical Ripening
In cases where the cervix is not ripe, the success rate of vaginal delivery after labor induction is lower. The Bishop score is the primary method used to evaluate cervical ripeness. A score of ≥7 suggests that labor induction may proceed directly, while a score of <7 indicates the need for cervical ripening prior to induction. Commonly used methods for cervical ripening include vaginal preparations of prostaglandin E2 (PGE2) and cervical dilator balloons.
Labor Induction
When the cervix is ripe, labor induction can be performed. Intravenous infusion of oxytocin is frequently used to initiate uterine contractions and achieve labor. If the fetal head is already engaged, artificial rupture of membranes is typically carried out first. Oxytocin infusion is initiated 1–2 hours later to continue the induction process. Artificial rupture of membranes facilitates the release of endogenous prostaglandins, enhancing the effectiveness of induction while allowing the assessment of amniotic fluid characteristics. Membrane sweeping may be considered before labor induction to reduce induction rates and the risk of postterm pregnancy, but it is generally not performed simultaneously with artificial rupture of membranes.
Labor Management
Postterm pregnancy is often associated with fetal distress and meconium-stained amniotic fluid, requiring appropriate preparations during delivery. Continuous fetal heart rate monitoring during labor is necessary, with attention given to the characteristics of amniotic fluid and fetal heart rate. Fetal distress requires early intervention. If meconium-stained amniotic fluid is detected, the neonate's vitality should be assessed immediately after delivery. For vigorous neonates, basic resuscitation may proceed as usual, while non-vigorous neonates require tracheal intubation and suctioning of meconium within 20 seconds to reduce the risk of meconium aspiration syndrome.
Cesarean Section
The risk of fetal distress is higher in postterm pregnancies, and the indications for cesarean section may be appropriately broadened in such cases.