A postmature infant is an infant born after 42 wk gestation.
The cause of postmaturity is generally unknown. Very rarely, it may be caused by abnormalities that affect the fetal pituitary-adrenal axis (eg, anencephaly or adrenal agenesis).
Past term, the placenta involutes, and multiple infarcts and villous degeneration cause placental insufficiency syndrome. In this syndrome, the fetus receives inadequate nutrients from the mother, resulting in soft-tissue wasting. During labor, postmature infants are prone to develop asphyxia; meconium aspiration syndrome, which may be unusually severe because post-term amniotic fluid volume is decreased and the aspirated meconium is less diluted; and neonatal hypoglycemia caused by insufficient glycogen stores at birth. Because anaerobic metabolism rapidly uses the remaining glycogen stores, hypoglycemia is exaggerated if perinatal asphyxia has occurred.
Symptoms and Signs
Postmature infants are alert and appear mature but have a decreased amount of soft-tissue mass, particularly subcutaneous fat. The skin may hang loosely on the extremities and is often dry and peeling. The fingernails and toenails are long. The nails and umbilical cord may be stained with meconium passed in utero.
Diagnosis is by clinical appearance (see Assessment of gestational age—new Ballard score.) and estimated date of delivery.
Prognosis and treatment depend on complications. Neonates with meconium aspiration may have chronic respiratory insufficiency and secondary pulmonary hypertension if untreated; surfactant replacement therapy is frequently helpful.
Last full review/revision March 2007 by James W. Kendig, MD
Content last modified February 2012