Triplet births are on the increase, even when IVF pregnancies are discounted, and the death rate for triplets is 10 times higher than for single births, Norwegian scientists said on Wednesday.
The researchers, who said their findings were likely to be similar in other parts of Europe, said the increase in triplet pregnancies was probably due to use of hormone drugs to stimulate ovulation and a rise in the average age of mothers.
The results mean more effort is needed to control such treatments to cut the number of triplet pregnancies, they said.
The study analyzed more than 2 million pregnancies between 1967 and 2006 in Norway and found that although death rates have fallen for singles, twins and triplets in the past 40 years, the rate for triplets is still 10 times higher than for single births.
"Advances in obstetric practice and perinatal care have increased the survival of very preterm infants," said Anne Tandberg of the Haukeland University Hospital in Bergen, who worked on the study. "However ... the improvements have not been so favorable for triplets compared to twins and singletons."
Tandberg said that although the study excluded births after in-vitro fertilization (IVF) -- which involves eggs being fertilized in a laboratory dish and the embryos being implanted into the mother -- the underlying triplet rate was still rising.
At about 2.7 per 10,000 pregnancies between 2002 and 2006, the rate of triplet pregnancies is almost two-and-a-half times higher than it was in the 1970s, the study found.
Philip Steer, editor of BJOG International Journal of Obstetrics and Gynecology which published the study, said it was important to try to cut the number of triplet pregnancies.
He noted that in Britain, the triplet birth rate fell after embryology authorities limited the number of embryos transferred in women under 40 during IVF treatment to two, more than halving the proportion of triplets born from 2000 to 2007.
He said hormone treatments should be more closely monitored or IVF embryo transfers reduced yet further to try to cut the number of triplet pregnancies.
"All hormonal treatment for ovulation induction should either be monitored more carefully ... or moved to IVF with single embryo transfer," Steer wrote in a comment on the study.
Tandberg said it was important also to try to prolong triplet pregnancies beyond the 28th week of gestation to give the babies the best chance of survival.
The fetal death rate before this stage is 50 percent, she said, and falls to just 3.8 percent beyond it.