Compared with term pregnancies, the morbidity and mortality rates for both mother and fetus increase several-fold with postterm pregnancy. Risks of maternal vaginal trauma, labor dysfunc-tion, and cesarean delivery increase. Cesarean delivery car-ries increased risks of infection, bleeding, thromboembolic phenomenon, and visceral injury. Stillbirth and neonatal mortality rates increase steadily after 37 weeks, approach-ing 1 in 300 at 42 weeks, and increasing several-fold as the 44th week approaches. It is impossible to discuss postterm gestations without discussing macrosomia, shoulder dysto-cia, meconium aspiration syndrome (MAS), dysmaturity syndrome, and oligohydramnios, as these comorbidities are closely related.
Macrosomia is defined as an abnormally large infant size,specifically, an infant weighing 4000 g to 4500 g or greater. It oc-curs in approximately 2.5% to 10% of postterm pregnancies. Maternal obesity, diabetes mellitus, or a previous macro-somic infant further raise the risk. Macrosomia is associated with an increased incidence of birth trauma, particularly if the infant is delivered vaginally. Such trauma includes shoul-der dystocia; fracture of the clavicle; and associated brachial plexus injury, specifically Erb–Duchenne palsy.
Shoulder dystocia is an obstetrical emergency causedby impaction of the anterior fetal shoulder behind the symphysis pubis during the process of vaginal delivery.
A series of particular maneuvers can be accomplished to re-lease this impaction. Brachial plexus injury is reported in approximately 0.85 to 1.89 per 1000 term deliveries, but increases 18- to 21-fold in macrosomic infants delivered vaginally; it can also occur during cesarean deliveries. In Erb-Duchenne palsy, paralysis, stretch, or tear injury tothe upper roots of the brachial plexus, at C5 and C6, results in paralysis of the deltoid and infraspinatus muscles and flexor muscles of the forearm, causing the limb to hang limply close to the side, with the forearm extended and in-ternally rotated; finger function is usually retained. Less frequently, damage is limited to the lower nerves of the brachial plexus, C8 and T1, causing Klumpke paralysis, or paralysis of the hand. Because most brachial injuries are mild, treatment is expectant, with splints and physical ther-apy in anticipation of complete or nearly complete recov-ery in 3 to 6 months. Eighty to 90% of brachial plexus injuriescompletely resolve by 1 year of age. Maternal risks with fetalmacrosomia include a two-fold risk of cesarean delivery— with its associated operative risks and maternal trauma, particularly involving perineal lacerations if the fetus is delivered vaginally.
Another special concern in postterm pregnancies is meconium passage and meconium aspiration syn-drome. MAS can lead to severe respiratory distress frommechanical obstruction of both small and large airways, as well as to meconium chemical pneumonitis. Meconium passage is not limited to postterm pregnancies, although prolonged pregnancy, particularly in the setting of oligo-hydramnios, is a substantial risk factor. Meconium passage occurs in 12% to 22% of women in labor, with aspiration occurring in up to 10% of these infants. The incidence of meconium passage increases as pregnancy becomes pro-longed, as does the incidence of meconium aspiration syndrome.
Dysmaturity syndrome, which refers to infants withcharacteristics resembling chronic growth restriction, affects up to 20% of postterm pregnancies.
These pregnancies are at increased risk of umbilical cord compression from oligo-hydramnios, meconium aspiration, and short-term neonatal complications (such as hypoglycemia, seizures, and respira-tory insufficiency) and have an increased incidence of non-reassuring fetal testing, both antepartum and intrapartum.
Oligohydramnios is defined as decreased amnioticfluid for gestational age, and is generally quantified as an amniotic fluid index less than 5 cm. This is measured by dividing the gravid abdomen into quadrants and totaling the measurements of the largest vertical pockets of fluid in each of those quadrants. Amniotic fluid is a reflection of fetal swallowing, fetal breathing, fluid transfer across the amniotic sac, and, especially, fetal urination. The amniotic fluid reaches its maximum volume at approximately 34 to 36 weeks, and stays constant or slightly decreases from there for the remainder of the pregnancy. Any alterations in the above processes can cause changes in amniotic fluid volume. Oligohydramnios is associated with poor outcomes sec-ondary to umbilical cord compression, uteroplacental insuffi-ciency, and meconium aspiration. Because of these risks, after40 weeks of gestation, close antepartum surveillance is warranted if pregnancy is allowed to continue. At term, oligohydramnios is an indication for delivery.