PRAGUE — Among babies conceived through assisted reproductive technologies, singletons that survive after the intrauterine demise of a twin have poorer obstetrical outcomes than do those that never had a twin.
The so-called vanishing twin effect can be seen in the surviving twin's smaller size for gestational age, lower birth weight, and younger gestational age at delivery, compared with singleton babies. Moreover, the adverse effects of the twin's demise rise with increasing gestational age, Dr. Anja Pinborg reported at the annual meeting of the European Society of Human Reproduction and Embryology.
She presented a retrospective cohort study involving 11 fertility clinics and 72% of the babies conceived via assisted reproductive technologies (ART) in Denmark between 1995 and 2001. Among these, 5,237 babies were singletons, and 3,678 were twins. The remaining 642 were survivors of a vanishing twin—in other words, two fetuses initially had been identified, but there was only one live birth.
A total of 66% of the vanished twins had disappeared before the eighth week of gestation, 29% had disappeared between 8 and 22 weeks' gestation, and among 5% the demise had occurred after 22 weeks and was classified as a stillbirth.
Surviving twins were more likely to be small for gestational age (SGA)—meaning a birth weight that was below the 10th percentile for that age, compared with singletons (5.3% vs. 3.6%). After controlling for maternal age, parity, and child gender, having a vanishing twin was the only significant predictor of SGA in singleton births (odds ratio 2.1). Among twins who had both survived, the rate of SGA was 13%, she said.
Survivors of a vanishing twin were more likely to be SGA if the twin's demise happened later in the pregnancy.
Similarly, mean gestational age was greater in singletons (39.5 weeks), compared with survivors.
And among babies delivered at term, low birth weight (defined as less than 2,500 g) was more frequent in survivors of a vanishing twin (3.8%), compared with singletons (2.3%), she said.
The magnitude of this effect increased with the gestational age at the time of the twin's demise.
“Vanishing twin is one of the reasons for poorer obstetrical outcome among ART singletons compared to singletons conceived naturally,” Dr. Pinborg explained, adding that this further supports the argument for elective single embryo transfer.