Very premature twins fare just as well as single babies born early, and they may even face a lower risk of certain complications.
For twin pairs of the same sex but sharply different sizes who are born before 28 weeks, the risks of death and bleeding on the brain are higher than they are for single babies born at the same time, Dr. Jennifer Zeitlin of the Hopital Saint-Vincent de Paul in Paris and her colleagues found.
Premature birth is much more common among twins than singletons, Zeitlin and her team note; while one in every 10 twin pairs is born before 32 weeks' gestation, just one in 100 singletons is born this early. There is evidence that preemie twins do better than singles of the same gestational age, they add.
A full-term pregnancy lasts for 39 weeks, while babies born between 28 and 31 weeks are considered "very pre-term." Babies born between 24 and 27 weeks' gestation are "extremely pre-term."
To investigate outcomes for very premature and extremely premature twins compared to those of singletons born equally early, Zeitlin and her colleagues looked at births and stillbirths in nine European countries in 2003. Their analysis included 1,254 twins and 3,586 singletons born between 24 and 31 weeks' gestation.
The women carrying twins were less likely to develop high blood pressure during pregnancy
than those with singletons, the researchers found: about 8 % of those carrying twins, compared to about 22 % of those carrying single babies.
They also found that severe bleeding and restrictions on the growth of the foetus were also less common in twin pregnancies.
Twins' mothers were also more likely to have been given corticosteroids before delivery; these drugs are administered to speed up premature newborns' lung development.
Among the very premature babies, the likelihood of dying in the first few weeks of life was lower for twins, who were also less likely to need oxygen. But once the researchers took factors such as mother's age, pregnancy complications, and infant health problems into account, the difference disappeared.
For the extremely premature infants, however, the researchers calculated that the risk of death or serious bleeding in the brain was about 1.5 times higher for twins than it was for single babies. While about 17 % of singletons suffered from such bleeding, roughly 24 % of twins did.
The greater risks were only seen for same-sex twins in which one twin weighed at least 15 % more than the other twin at birth.
"Why the effects of these twin-specific complications were so much more pronounced for extremely preterm births is an area for further study," the researchers conclude.