Ability to predict complications during pregnancy depends partly on knowing early whether a woman is carrying identical twins. Ultrasound can produce and refine that knowledge, according to Australian researchers.
First-trimester ultrasound can determine chorionicity but not zygosity. Using transvaginal ultrasound, investigators sought to determine zygosity in 33 twin pregnancies. By observing the number of corpus lutea in the ovaries, they found they could tell identical from fraternal twins. Findings were published in the January issue of Ultrasound in Obstetrics and Gynecology.
The method may be able to determine twins' zygosity more than 30 weeks before birth, said principal investigator Dr. Stephen Tong, an ob/gyn clinician at Monash University in Clayton, Victoria.
The ultrasound technique could establish whether twins are identical (monozygotic) or nonidentical (dizygotic) in almost all cases of spontaneous twin pregnancy between four and nine weeks of gestation, Tong said.
Chorionicity agreed with the predicted zygosity in all 33 cases in the study. Researchers found one corpora luteum in 15 cases, which were designated as monozygotic. Among these, four were classified as monochorionic (sharing one placenta) and 11 as dichorionic. Researchers observed two corpus lutea in the remaining 18 cases. They were designated as dizygotic and classified as dichorionic.
Monochorionic twins are at the highest risk of complications during pregnancy, according to the clinical literature. Their perinatal mortality rate is three times higher than that of dichorionic twins. They face a higher rate of twin-to-twin transfusion syndrome and are more likely to suffer from congenital anomalies such as conjoinment.
Though encouraging, the findings require verification. The case number was low, mostly because researchers excluded twins conceived in vitro and included only spontaneous twins, considered a relatively rare event.
Along with several potential clinical applications, social and psychological implications should also be accounted for, Tong said. The International Society for Twin Studies, founded in Rome in 1974, states that parents have a right to expect accurate recording of placentation zygosity diagnosis of same-sex twins. Their similar biology affects their development, and they are blood and organ donors of choice for their siblings.
"Twins have a right to know their zygosity, and it is likely that they use this information to shape their identities," Tong said.
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