New research to be published in BJOG: An International Journal of Obstetrics and Gynaecology reveals how the rate of births from triplets has increased, even after excluding pregnancies that were the result of assisted reproductive technology (ART) in the form of in vitro fertilisation (IVF).
Norwegian researchers examined the records of over 2 million pregnancies in the Medical Birth Registry of Norway between 1967 and 2006, to compare births from triplet pregnancies with singleton and twin pregnancies. All reported live births and stillbirths from the 16th week were looked at. Triplet births are known to be associated with higher adverse pregnancy outcomes when compared to singleton and twin births and this was tested by observing trends in birth weight, gestational age and perinatal mortality. Although perinatal mortality has fallen over the last forty years for singletons, twins and triplets, the perinatal mortality for triplets remains 10 times higher than that for singleton births.
The study focused on two periods: 1967 – 1987 and 1988 – 2006 (after ART was introduced) and these were further divided into five-year blocks. The peak incidence of triplet births were between 1987 and 1991 (3.5/10,000 pregnancies). This decreased to 2.7/10,000 between 2002 and 2006, probably as the result of the introduction of new national clinical guidelines recommending a policy of single embryo replacement at IVF (with which all fertility clinics in Norway have adhered). However, the rate of triplet pregnancies remains almost 2.5 times higher than it was in the 1970s, most likely due to the use of ovulation inducing drugs (such as gonadotrophins and clomiphene) which are not recorded as ART in the official figures, and the increase in average maternal age.
Maternal age for triplet pregnancies increased by an average of 2.5 years when comparing both time periods while the caesarean section rate in triple pregnancies increased from 46.7% to 92%. The gestational age of triplet births fell from 34.1 weeks in the first period to 32.1 weeks in the second period. This was matched by a fall in the birth weight of triplets over the two time periods. However, the perinatal mortality rate for triplets was 13.6% before the introduction of ART and 7.1% afterwards, reflecting the improvements in neonatal care. This fall only paralleled that in singletons, and the comparative ten-fold higher risk of death for triplets remained unchanged.
The Norwegian data showed, in agreement with most other studies, that the fetal growth rate for triplets slows in the third trimester when compared to singletons. This is the result of competition between the fetuses for nutrients in the womb. Researchers also note that modern obstetric practice recommends early intervention for triplet births and the improved perinatal mortality rate observed confirms the appropriateness of this practice.
Co-author Dr Anne Tandberg, from the Department of Obstetrics and Gynaecology at Haukeland University Hospital in Bergen, Norway said, “In our study, we found a ten-fold higher risk for triplet infants dying in pregnancy until the first week of life when compared to singletons.
“Advances in obstetric practice and perinatal care have increased the survival of very preterm infants. However, our results show that the improvements have not been so favourable for triplets compared to twins and singletons.
“Our study findings show that it is very important to prolong the triplet pregnancy beyond the 28th week of gestation. The mortality rate below versus above this cut-off point is 50% and 3.8 % respectively.”
Professor Philip Steer, BJOG editor-in-chief said, “This research shows that the perinatal mortality rate for triplet births fell in recent years and a similar trend was seen in last year’s CEMACH Perinatal Mortality Report. This is good news and down to the careful management of multiple pregnancies but other factors such as a fall in the birth rate of triplets in recent years have a part to play.
“Unfortunately, the rate of triplet pregnancies continues to be much higher than it was before the introduction of ovulation induction and the trend to giving birth at an older age. In the UK, the triplet birth rate fell after the Human Fertilisation and Embryology Authority (HFEA) limited the number of embryos transferred in women under 40 during IVF treatment to two. This has effectively more than halved the proportion of triplets born from 0.04 in 2000 to 0.02 in 2007.
“To reduce the number of triplet pregnancies further, all hormonal treatment for ovulation induction should either be monitored more carefully with cancelling of multifollicular cycles, or moved to IVF with single embryo transfer.”
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Tandberg A, Bjørge T, Nyga°rd O, Børdahl P, Skjaerven R. Trends in incidence and mortality for triplets in Norway 1967–2006: the influence of assisted reproductive technologies. BJOG 2010; DOI: 10.1111/j.1471-0528.2010.02530.x.
To view an abstract of the paper, click here