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SAN FRANCISCO – The physician is responsible for knowing the chorionicity of a pregnant patient’s twins, so if you don’t offer diagnostic ultrasound or can’t make the diagnosis yourself, refer the patient to someone who can.
Knowing the chorionicity is essential because "it literally will define how you manage the pregnancy," Dr. Larry Rand said at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.
Courtesy Dr. Larry Rand
These monochorionic/diamniotic twins at 9 weeks' gestation have a single placental mass and a thin, wispy membrane separating the twins, with no chorionic (placental) tissue between the sacs.
"This is a critical, critical concept," said Dr. Rand, director of perinatal services for the Fetal Treatment Program at the university. The chorionicity "should be the very first question that you ask yourself when you have a patient with twins."
You don’t need to be an expert on monochorionic twins; you just need to know if the patient is at high risk because of monochorionicity. Determining the chorionicity of twins before 14 weeks’ gestation is considered the standard of care, he stressed, and a physician who doesn’t know that twins are monochorionic could be liable if something goes wrong.
If the chorionicity is undetermined on your office ultrasound, refer the patient and request a chorionicity determination, he advised.
"This is one of the times that I have to say that ultrasound makes all the difference," Dr. Rand said. "The single most important ultrasound finding in the entire pregnancy is going to be the chorionicity."
A patient deserves to know whether her twins are monochorionic because she needs to be counseled appropriately about the risks. "It’s not the radiologist’s responsibility," he said. "The obstetrician is responsible for knowing the effect of chorionicity." Make sure to document that you have either determined the chorionicity yourself or have asked for an exam to assess chorionicity.
Courtesy Dr. Larry Rand
These dichorionic/diamniotic twins at 10 weeks' gestation have two placental masses abutting one another, with a "twin peak" sign -- visible chorionic tissue between the two sacs (which would not be seen if the two placentas were on opposite surfaces instead of adjacent to each other).
The rate of congenital anomalies in monochorionic twins, for example, is similar to the rate seen in diabetic mothers. This "changes the kind of tests you order and the things you see" compared with dichorionic twins, he said. Level II ultrasound examinations of anatomy that would be done in high-risk cases such as diabetic pregnancies, for example, should be done for monochorionic twins, whose elevated risk for anomalies may be due to an imperfect splitting of the single egg from which they came.
Compared with dichorionic twins, monochorionic twins also have increased risk for neurologic injury, including an eightfold increased incidence of cerebral palsy, and for preterm birth, spontaneous abortion, intrauterine growth restriction and growth discordance, intrauterine fetal demise, and twin-to-twin transfusion syndrome. Specialized centers can treat twin-to-twin transfusion syndrome with laser therapy if patients are diagnosed and referred in a timely manner – which is the reason for more-frequent screening of monochorionic twins. (See management recommendations below.)
Aterio-arterial (AA) anastomosis, a type of vascular connection within the placenta, can be detected on antenatal ultrasound. If present, the risk of twin-to-twin transfusion syndrome decreases 10-fold and the risk of intrauterine fetal demise decreases sevenfold. "It’s very helpful prognostically," he said.
Although AA anastomosis is not a new concept, it’s only recently that obstetrical imaging has evolved to identify it on antenatal ultrasound, which is "so helpful for counseling and management," said Dr. Rand, also the Lynne and Marc Benioff Endowed Chair in Maternal and Fetal Medicine at the university.
The major fear in monochorionic twins is intrauterine fetal demise of one twin, which leaves the surviving twin with a 10%-20% risk of death and a 20%-40% risk of long-term neurologic injury if the second twin survives. The difference even comes into play with genetic counseling. Aneuploidy risk in monochorionic twins is similar to risk in singletons (because they originated from a single egg). For invasive testing, one chorionic villus sampling covers both monochorionic twins (because there’s only one placenta to sample), but amniocentesis typically is performed on each amniotic sac. Some physicians sample just one amniotic sac for amniocentesis, thinking that it should cover both twins because they’re identical, but there are extremely rare cases of subtle differences that would not be detected by single-sac sampling, he said.
Dr. Larry Rand
The other outcomes differ between monochorionic and dichorionic twins because the splitting of an egg can be imperfect; monochorionic twins share "plumbing" and vascular connections, and they have to share the resources of one placenta, Dr. Rand explained.