Late-term and postterm pregnancy

Summary

A late-term pregnancy is defined as 41 0/7 to 41 6/7 weeks' gestation; at 42 weeks' gestation, it becomes a postterm pregnancy. While the causes of postterm pregnancy are generally unknown, many supposed cases are likely due to a suboptimally dated gestational age. Risk factors may be maternal (e.g., age, previous postterm pregnancies, nulliparity) and fetal (e.g., male sex, disorders such as oligohydramnios). Patients with low-risk late-term pregnancies who have not had suboptimal gestational dating and who do not have indications for cesarean delivery or contraindications for induction of labor are usually offered the choice between expectant management with antepartum fetal surveillance and induction of labor. After 42 weeks' gestation, induction of labor is recommended regardless of cervical favorability in order to avoid associated complications. Membrane sweeping may be performed in well-dated pregnancies after 39 weeks' gestation to induce labor and prevent late-term or postterm pregnancy.

Definition

• Postterm: ≥42+0 weeks
• Late term: 41+0 to 41+6 weeks
• Full term: 39+0 to 40+6 weeks
• Early term: 37+0 to 38+6 weeks

Epidemiology

• 0.25% of pregnancies exceed 42 weeks.
• Postmaturity syndrome occurs in < 20% of postterm pregnancies.
• Variations exist across populations, influenced by factors like early ultrasound use, which can significantly reduce postterm pregnancy rates by providing accurate gestational dating.

Etiology

• Idiopathic
• Wrong dating
• Rarely, hormonal defects in the fetus can delay labor onset.

Risk Factors

• Previous postterm pregnancy presents the highest risk
• Nulliparity
• Male fetus
• Obesity
• Older maternal age
• Specific maternal ethnicities

Diagnosis

• Based on the most accurate gestational dating, preferably by early ultrasound, to align the estimated due date with sonographic findings.

Morbidity and Mortality

• Postterm pregnancies are prone to macrosomia and dysmaturity, increasing the risks of labor complications, umbilical cord compression, and fetal hypoxia.
• Perinatal mortality rates double after 42 weeks.

Management

• Induction at 41+0 weeks is favored to reduce perinatal mortality without increasing morbidity or cesarean rates.
• Expectant management with twice-weekly fetal monitoring is an alternative, allowing for spontaneous labor while closely watching for signs of fetal distress.

Recurrence Risk

• After one postterm delivery, the likelihood of another increases significantly, highlighting the importance of careful monitoring in subsequent pregnancies.

Prevention

• Early routine ultrasound and term membrane sweeping can reduce the incidence of postterm pregnancies by improving gestational dating accuracy and possibly initiating labor.
• Membrane sweeping
  • Definition: a procedure involving manually separating the amniotic membranes from the lower uterus wall during a pelvic examination
  • Timing: may be performed at or after 39 0/7 weeks' gestation
  • Goals:
    • Promote the onset of normal spontaneous labor
    • Decrease the risk of late-term and postterm pregnancy and need for induction
  • Risks include:
    • Vaginal bleeding
    • Pain
    • Irregular contractions

Conclusion

Post-term pregnancies carry increased risks for both the mother and baby, including higher rates of macrosomia, fetal dysmaturity, and perinatal mortality. Early accurate dating with ultrasound and timely induction strategies can mitigate these risks, enhancing outcomes. The choice between induction at 41+0 weeks and expectant management with close fetal monitoring should be personalized, considering the patient's preferences and clinical scenario.