20.3 Post-term Birth

Post-term birth is defined as infants born after 42 weeks’ gestation. The incidence of post-term pregnancy is about 7% of all pregnancies. Risk factors for post-term pregnancy include primiparity (first pregnancy), previous post-term pregnancy, male fetus, obesity, hormonal factors, and genetic predisposition.^[1]

Post-term pregnancy causes increased risk of fetal mortality and morbidity that are even higher than risks associated with preterm birth. The risk of intrauterine fetal demise, commonly called stillbirth, increases after 39 weeks’ gestation, with a sharp rise after 40 weeks’ gestation. Conditions related to post-term birth are meconium aspiration syndrome, macrosomia, and dysmaturity syndrome^[2]:

• Meconium aspiration syndrome: Fetal aspiration of meconium-stained amniotic fluid can result in respiratory distress that is further discussed in the following subsection.
• Macrosomia: A newborn who weighs more than 8 pounds, 13 ounces (4,000 grams), regardless of their gestational age, is diagnosed with macrosomia. Post-term newborns may also be large for gestational age (LGA), meaning they are in the 90th percentile or higher for their weight compared to other infants of the same age. LGA can result in complications such as shoulder dystocia or birth asphyxia. Additional information about LGA is discussed under the “Birth Weight” section.
• Dysmaturity syndrome: Approximately 20 percent of post-term infants have dysmaturity syndrome with chronic intrauterine growth restriction, resulting from uteroplacental insufficiency. These pregnancies are at increased risk for umbilical cord compression from oligohydramnios, as well as nonreassuring fetal heart rate patterns, neonatal seizures, neonatal hypoglycemia, and increased risk of infant death within the first year of life. In addition to these newborns being small for gestational age (SGA), other signs of dysmaturity are thin, wrinkled, peeling skin; long hair and nails; and meconium-stained amniotic fluid.^[3],[4] Additional information about SGA is discussed under the “Birth Weight” section.

Health care providers make decisions about inducing labor or performing cesarean delivery in post-term pregnancies based on the clinical status of the fetus and the mother. Fetal surveillance may include a biophysical profile (BPP) that includes a fetal ultrasound and nonstress test (NST). The ultrasound assesses fetal movements, amniotic fluid volume, muscle tone, and the fetal heart rate. An NST evaluates the fetal heart rate reactivity and responsiveness. A nonreactive NST may indicate fetal distress related to fetal hypoxemia.^[5],[6]

Meconium Aspiration Syndrome

Post-term and small for gestational age (SGA) newborns are at risk for meconium aspiration syndrome (MAS), which is diagnosed when respiratory distress occurs in a newborn delivered with meconium-stained amniotic fluid and no other underlying reason for respiratory distress. Meconium-stained amniotic fluid is visible at delivery and can also cause staining to the newborn’s vernix, umbilical cord, and nails.^[7]

Meconium is a dark green substance that typically forms a newborn’s first feces. However, meconium present in the amniotic fluid is considered a sign of fetal distress. It is usually caused by decreased fetal oxygenation that triggers a fetal reflex to expel bowel contents, which can then be inhaled during delivery.^[8] See Figure 20.25^[9] for an image of meconium in a diaper.

Signs of MAS include respiratory distress with tachypnea and cyanosis. The newborn often has a barrel-shaped chest with increased anterior-posterior diameter due to hyperinflation of the lungs. In severe cases, pneumothorax (collapsed lung with air in the thoracic space) can lead to respiratory failure. MAS is diagnosed by chest X-ray that demonstrates streaky, linear densities and a flattened diaphragm due to hyperinflation of the lungs.^[10]

Prevention is the best management for MAS, but when prevention fails, treatment is similar to any newborn with respiratory distress. Inadequate respiratory effort resulting in gasping, increased work of breathing, and decreased oxygenation are treated with respiratory support, which may include oxygen therapy and intubation and mechanical ventilation. However, guidelines from the American Heart Association (AHA), the American Academy of Pediatrics (AAP), and the American College of Obstetricians and Gynecologists (ACOG) recommend against routine nasopharyngeal suctioning when meconium is suspected.^[11]

A potential complication of meconium aspiration syndrome is persistent pulmonary hypertension of the newborn.^[12]