Twins complicate approximately 1:90 pregnancies and pose challenging antenatal management issues. I will briefly discuss
our current recommendations with regards to prenatal screening, growth evaluations, and antenatal testing.
Early diagnosis and determination of gestational age is essential to maximizing perinatal outcome. Aneuploidy screening and serial growth evaluations will be based on the accuracy of the gestational age. First trimester, second trimester and integrated screening for aneuploidy is available for twin pregnancies but are only offered by a few specialty laboratories.
The placentation type will also dictate perinatal management. The chorionicity of the placenta is best determined sonographically in the early second trimester of pregnancy. Diamniotic/ dichorionic (di/di) placentas are the most common. In most but not all cases, the pregnancy is a result of dizygotic (DZ) twining. Since the placentas are separate, these pregnancies are not at risk for twin-twin transfusions syndrome (TTTS). However, there is an increased risk for intrauterine growth retardation particularly in the third trimester of pregnancy. Ultrasound evaluations for growth every 4 week in the third trimester of pregnancy is recommended. Antenatal testing is not required for appropriately growing di/di twin pregnancies. Delivery is expected at 37 weeks gestational age on average.
Diamniotic/monochorionic (di/mo) placentations only occur in monozygotic (MZ) twins and constitute 70% of MZ twin placentas. Of these pregnancies, 10% will develop TTTS. The hallmark of this dangerous disorder is discordance in the amount of amniotic fluid volume. No longer is fetal weight discordance useful for the diagnosis. Once the diagnosis has been confirmed, the pregnancy is staged according to the Quintero staging system which was developed to follow the progression of the disease and establish prognosis. TTTS that presents before 28 weeks and is untreated will result in a 90% perinatal mortality rate. Based on both European and US studies, the optimal treatment for this disorder appears to be laser therapy. There are several centers offering this treatment in the mainland US. A TTTS diagnosis made after 26 weeks gestation does not meet criteria for laser therapy. Serial amniocenteses can be considered for these patients.
Another 10% of di/mo pregnancies will develop selective IUGR of one twin. This is different than TTTS and is defined by IUGR of one fetus, abnormal umbilical artery Doppler blood flows, and non-discordance of the amniotic fluid volume. Approximately 40% will result in spontaneous death of a twin. The resultant drop in blood pressure will cause significant brain damage or death of the remaining twin 70% of the time. Treatment options have been limited to expectant management, umbilical cord occlusion, or termination of the pregnancy. Recently, a multicenter prospective randomized clinical trial has been initiated comparing expectant management with laser therapy. Data from this trial are not yet available.
All di/mo twins are followed closely with ultrasound examinations at every 2-4 week intervals. Since TTTS can develop at any time, it is also recommended that these patients be followed with antepartum testing beginning at 28 weeks. Delivery should be accomplished at 37 weeks gestation.
Monoamniotic placentations are both the most rare and dangerous of the MZ twin pregnancies. Because of cord entanglement, the perinatal loss rate has been estimated to be as high as 50-60%. Since cord occlusion is largely unpredictable, continuous or daily fetal monitoring should be initiated at the time of reasonable fetal viability (26-28 weeks). After celestone administration, delivery by elective cesarean section at 32-34 weeks is recommended.
These are some basic guidelines for the management of twin pregnancies. For more specific recommendations, please feel free to call our office.
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