A number of fetal conditions place the fetus at risk for stillbirth, hypoxia/acidosis, and/or cardiac failure. Examples include fetal growth restriction, congenital malformations, multiple gestations and isolated oligohydramnios. Late preterm birth or early term birth may be beneficial under such circumstances to avoid further fetal risk secondary to a progressively, or even stable, hostile intrauterine environment.
Fetal growth restriction in singleton gestations has been variably defined using birth weight cutoffs of less than the third, fifth or 10th
percentiles. Since many of these fetuses are constitutionally small, and because fetuses can experience growth failure despite being larger than an arbitrary cutoff, some investigators have advocated incorporation of maternal and paternal characteristics to derive “individualized fetal growth assessment” as a more predictive marker of fetal risk from poor intrauterine growth (29
). In twin gestations, fetal growth restriction has been defined using standard cutoffs similar to singletons or growth curves specific to twins, with either method identifying about 20–25% of twin fetuses to be below the 10th
percentile for all fetuses. Additionally, discordance in fetal size between twins of 20% or more is used to delineate a fetus that is not reaching its potential for intrauterine growth. The major aim of early delivery in the setting of fetal growth restriction is to minimize fetal and newborn risks, particularly fetal death and long term neurologic sequelae. The presence of concurrent medical or obstetrical conditions may shift the optimal timing of delivery towards an earlier gestation ().
Over 2% of pregnancies are complicated by major congenital anomalies, and 1% have anomalies that require delivery planning. The risks of complications dependent on the particular anomaly. Examples of maternal complications include anxiety, pregnancy-related hypertension, mirror syndrome, polyhydramnios (with its risks of respiratory compromise, abruption placentae or preterm labor), PROM or maternal respiratory compromise. The optimal gestational age of delivery to minimize fetal and newborn complications cannot be generalized as some fetal abnormalities require expeditious delivery while others can be managed expectantly. Many fetal anomalies do not necessitate either late preterm or early term birth, and these fetuses are better served by allowing time for growth and maturation in-utero. Some overarching recommendations are provided in .
Multiple gestations, including dichorionic/diamniotic and monochorionic/diamniotic twins, whether uncomplicated or affected by a single fetal death, carry increased maternal and fetal risks. Three percent of deliveries are twins and 50% of these deliver preterm with 30% delivering in the late preterm period. In addition to preterm birth, twin pregnancies are at increased risk for fetal growth restriction, congenital anomalies (monochorionic twins) as well as issues specific to multiples (twin-twin transfusion syndrome [3–4%], fetal death [1–2% per week in monochorionic gestations after 32 weeks] and discordant fetal growth). In addition, maternal conditions such as gestational diabetes, preeclampsia, abruptio placentae, placenta previa, and postpartum hemorrhage are more common in twin pregnancies. Early delivery may reduce the risk of stillbirth, and allow timing of delivery to optimize staffing and resources for newborn care in complex cases. Uncomplicated dichorionic twins have optimal outcomes when delivered at 38 weeks. Uncomplicated diamniotic-monochorionic twins have higher risks of stillbirth, thus a late preterm delivery (34–37 weeks) is recommended. Because of the high risk of cord entanglement resulting in fetal death in monoamniotic twins, delivery at 32 to 34 weeks is recommended. In the setting of a multiple gestation with one fetal demise, early delivery may prevent a second fetal death. In dichorionic pregnancy with a single fetal demise, expectant management to 37 weeks with weekly surveillance is generally recommended; however due to limited data, delivery between 34 and 36 weeks may also be reasonable. In monochorionic (mono- and di-amniotic) gestations with a single fetal demise, neurological injury may not be preventable due to the acute cardiovascular effects in the surviving co-twin, however early delivery might avert stillbirth in the live fetus. Regardless of chorionicity, maternal anxiety in these situations cannot be underestimated and may be a factor in the clinical judgment regarding the timing of delivery. Considerations regarding delivery timing for multiple gestations are presented in .
Isolated oligohydramnios has been defined by some as a single vertical pocket of 2.0 cm or less and by others as an amniotic fluid index of 5.0 cm or less. Using the single vertical pocket, oligohydramnios has been reported in 2.3% of pregnancies at 34–36 weeks and in 3% at 37 weeks or more. Using the amniotic fluid index, oligohydramnios is present in 4.8% of pregnancies at 34–36 weeks and 10% at 37 weeks or more. Oligohydramnios is associated with non-reactive non-stress tests (1.5-fold increase), fetal heart rate decelerations (1.8-fold), fetal intolerance in labor, and stillbirth (4.5-fold), as well as Apgar score ≤3 at 5 minutes (11-fold) and meconium aspiration (12-fold). Oligohydramnios in the presence of normal fetal growth may be less ominous than when it is associated with abnormal fetal growth. However, ultrasound is insensitive in the diagnosis of fetal growth restriction when population-based nomograms are used. The advantage of early delivery is the avoidance of stillbirth, but the maternal risks of early delivery are those related to labor induction and cesarean delivery. The optimal definition of oligohydramnios has not been determined. There is no consensus on which method to use in defining oligohydramnios in the context of timing of delivery. There is no evidence that one method is a better predictor of adverse neonatal outcome than the other. While either has been used to diagnose oligohydramnios, the single deepest pocket has higher specificity in the preterm period and would lead to lower rates of delivery (30
). In the setting of otherwise uncomplicated isolated and persistent oligohydramnios, delivery at 36–37 weeks is recommended (). Whether a different threshold would be needed if the single deepest pocket versus the AFI is used remains unknown. Given that amniotic fluid evaluation is a component of fetal surveillance tests, it would be prudent not to make the decision regarding timing of delivery for oligohydramnios in isolation and to include the results of the complete fetal testing (BPP or modified BPP) along with other clinical parameters such as maternal condition and fetal growth.