A new study published in the journal JAMA Pediatrics indicates that the optimal time of birth for twins is at 37 completed weeks of pregnancy.
Twins are now being born more often because of an increase in the use of assisted reproductive techniques, in which almost a quarter result in twinning. Though only about 3% of live-born babies are twins, about 15% of newborns who need admission, and babies in special care units, are twins. Moreover, twins are more often born by Cesarean section than singletons at corresponding periods of gestation. As a result of all these factors, twin health costs about three times as much as for singletons. The death rate for twins is also higher around the time of birth.
It is therefore vital to know the best time of delivery for twins – a matter which is, strangely, still largely undefined. Most guidelines today operate based on observational studies because of the randomized controlled trials seeking to define the optimal timing of birth. Overall, it is considered that twin delivery should be around 37 weeks for dichorionic twins and 36 weeks for monochorionic twins.
With singletons, the neurodevelopmental outcomes are best with delivery at 41 weeks, but the odds of death around the time of delivery are best at weeks 38-39. The current study was motivated by the need to understand how short-term and permanent outcomes are affected by gestational age.
The current cohort study analyzed the outcomes of over 43 000 twins born at 34 weeks or higher between January 1, 1980, and December 31, 2015, in Scotland. The various relevant registers were used to retrieve the required data – the Scottish Morbidity Record SMR02, the Scottish Stillbirth and Infant Death Survey (SSBID), and the Scottish Exchange of Educational Data (ScotXed). This enabled educational records to be associated with the Morbidity Record.
The researchers were looking for extended perinatal mortality, which means death by stillbirth or in the newborn period, and for SEN, special education needs. According to the UK Department of Education, a child with SEN is “unable to benefit fully from school without help beyond that normally given to schoolchildren of the same age.” The investigators included in this category conditions like intellectual disabilities, physical disability, autistic spectrum disorder, language or speech disorders, and psychological or social difficulties.
Boys and girls were equally represented in the analysis. The most significant percentage of twins (49%) was observed to be delivered at 37-38 weeks, and 6% were preterm. About 60% of mothers were between 25 and 35 years of age, and in 57%, this was their first pregnancy.
Twins had increased perinatal mortality, with the odds being over 2.5 times higher at 34-37 weeks compared to birth after that up to 39 weeks. When competing risks were analyzed, stillbirth risk was markedly less than the risk of neonatal death at 34 and 35 weeks, while the reverse held good after 37 weeks. At 37 weeks, both were equal.
The twins born before 38 weeks were more likely to die in the newborn period compared to infants who were delivered after this period. The adjusted odds ratio for newborn death for twins was 16 times higher at 34 weeks, and progressively reduced from about 5.6 times, to 1.5 times higher from 35 weeks to 37 weeks.
Twin children born before 37 weeks had higher odds of SEN at 34 – 36 weeks, but not after that. And the overall rate of SEN is higher for twins than for singletons.
The findings of this study point to 37 completed weeks of gestation as the best time for the delivery of twin infants, for both short-term and long-term outcomes. This is the first time that data has been obtained which supports the current clinical guidelines. The birth of twins before 37 weeks is associated with increased perinatal mortality and increased odds of SEN at school. The lowest SEN occurs at 37 weeks and beyond with twins.
A limitation of this study is the lack of sufficient sample size beyond 39 weeks. Similarly, the study does not help to determine the effect of the death of one twin in utero before delivery. Still, available data indicates this to be a rare problem and one that has to be addressed based on individual cases.
The researchers say, “This is the first nationwide study to investigate long-term educational outcomes of twins across the range of gestational age categories.” The rate of SEN is higher among twins, at about 14%, compared to 5% in singletons. They conclude, “In terms of short- and long-term outcomes, uncomplicated twin pregnancies should not be delivered before 37 completed weeks of gestation and that there is a limited benefit of prolonging pregnancy after that.”
Murray, S., MacKay, D., Stock, S., Pell, J., and Norman, J. Association of gestational age at birth with risk of perinatal mortality and special educational need among twins. JAMA Pediatrics 2020. doi:10.1001/jamapediatrics.2019.6317