2016 Update on obstetrics

Society for Maternal-Fetal Medicine, Simpson LL. Twin-twin transfusion syndrome. Am J Obstet Gynecol. 2013;208(1):3–18.

Society for Maternal-Fetal Medicine. Checklists and Safety Bundles. https://www.smfm.org/mfm-practice/checklists-and-safety-bundles. Published March 2015. Accessed December 7, 2015.

American College of Obstetricians and Gynecologists. Practice bulletin No. 144. Multifetal gestations: twin, triplet, and higher-order multifetal pregnancies. Obstet Gynecol. 2014;123(5):1118–1132.

From the maternal perspective, twin pregnancies are known to have higher risks than their singleton counterparts for such complications as hypertension, preeclampsia, diabetes, hemorrhage, cesarean delivery, postpartum depression, and anemia. These complications are managed essentially the same way regardless of the number of fetuses.

From the fetal/neonatal perspective, twin gestations may carry increased risks of congenital anomalies, preterm birth, and aneuploidy, which are managed similarly to singleton gestations overall, with certain adjustments as necessary.

Twin pregnancies do have unique risks, however, that are managed differently from the time chorionicity is established until delivery. The level of risk increases as the number of chorions and amnions decreases.

A basic management plan for twin gestations consists of a number of components, elucidated below.

1. Determine chorionicity and amnionicityThis determination is most reliably performed late in the first trimester and must be done using ultrasound. The inter-twin membrane should be identified. At 11 to 14 weeks, the presence of the “lambda sign,” a triangular projection of tissue that extends from the chorionic surface, is indicative of a dichorionic pregnancy, while a “T sign” suggests a monochorionic pregnancy (sensitivities 97%–100%; specificities 98%–100%). Alternatively, demonstration of discordant genders or separate placentas may be used later in pregnancy.

2. Monitor growth every 4 weeks in dichorionic twinsDichorionic twins are, by default, diamniotic. After the anatomic survey, growth surveillance should be conducted approximately every 4 weeks.

Discordant growth usually is defined as a difference of 20% or more in weight between the twins, based on the weight of the larger twin. As an isolated finding with both fetuses of normal weight, this discordance has not been demonstrated to increase adverse outcomes. Routine antenatal surveillance is not necessarily indicated.

Fetal growth restriction of one twin or a coexisting abnormality should prompt antenatal testing and/or earlier delivery. Any maternal comorbidities such as hypertension or diabetes also would be indications for testing. Otherwise, delivery is recommended at 38 weeks’ to 38-6/7 weeks’ gestation.

After 32 weeks, the mode of delivery may be vaginal if the presenting twin is vertex and the delivery provider can perform breech extraction or internal podalic version, if necessary.

3. Monochorionic/diamniotic twins also warrant regular surveillanceThe shared placenta places these pregnancies at increased risk for twin-to-twin transfusion syndrome (TTTS), a fetal-placental imbalance in which one twin “transfuses” the other. Ten percent to 15% of monochorionic pregnancies develop TTTS, which is associated with high rates of morbidity and mortality, even when treated.

Antenatal surveillance of these pregnancies involves ultrasonography assessment every 2 weeks, starting at 16 weeks. At each examination, the deepest vertical pocket (DVP) of fluid and presence of each fetal bladder are documented. This limited assessment alternates with a growth assessment every 2 weeks. SMFM recommends this biweekly assessment until 28 weeks, then every 2 to 3 weeks until delivery.

Stage 1 TTTS is defined by the polyhydramnios/oligohydramnios sequence (DVP of one fetus <2 cm, with DVP of the other >8 cm).

Evaluation for treatment of TTTS with laser coagulation (preferred) or amnio‑reduction should take place after the diagnosis is made, along with increased fetal surveillance.

SMFM also recommends fetal echocardiography due to the 9-fold increased risk of cardiac anomalies in monochorionic pregnancies.

Other complications of monochorionic/diamniotic twins include selective fetal growth restriction (due to unequal sharing of the placenta), twin reversed arterial perfusion (TRAP) sequence, and twin anemia-polycythemia sequence (TAPS).

Antenatal surveillance of all monochorionic twins is recommended, given the increased risk of stillbirth; many centers start testing at 32 weeks’ gestation. According to ACOG, uncomplicated monochorionic/diamniotic twins should be delivered at 34 weeks’ to 37-6/7 weeks’ gestation. Fetal growth restriction or other comorbidities may prompt delivery as early as 32 weeks.

4. Know the risks of monoamniotic twin gestationsThese twins are at increased risk for intrauterine fetal death due to cord entanglement, as well as TTTS, TAPS, and fetal growth restriction. Routine growth assessment and evaluation for TTTS are similar to those for monochorionic/diamniotic twins (without the option of polyhydramnios/oligohydramnios measurement), but the overall management of these pregnancies is unknown.

Protocols may range from outpatient antenatal testing to hospitalization to 24 to 28 weeks’ gestation with daily antenatal testing or attempted continuous monitoring. Delivery by cesarean delivery is recommended at 32 to 34 weeks’ gestation.

What this EVIDENCE means for practice Monochorionic twins need specific and frequent monitoring due to significantly increased risk for both fetal and placental complications. They justify late preterm or early term delivery.