The first results of a major all-Ireland study of twin pregnancies, led by Perinatal Ireland and RCSI and funded by the Health Research Board, has shown that a major difference in the birth weights among a pair of twin babies results in an increased risk of health complications for both twins. 1,001 women in Ireland with twin pregnancies between 2007 and 2009 participated in this study.

The study revealed that a difference of 18% or more in twin birth weights is associated with an increased risk of fetal or neonatal death, bowel complications, breathing difficulties, infection and admission to the neonatal intensive care unit. Twins exceeding this birth weight difference are known as a ‘discordant' pair. Twins who shared a single placenta, ‘monochorionic' twins, were found to be at the highest risk and the larger twin was at equal risk of morbidity as his/her smaller co-twin.

Dr Fionnuala Breathnach, Consultant Obstetrician at the Rotunda Hospital and Senior Lecturer in Maternal Fetal Medicine at RCSI is lead researcher for the study, which is published in Obstetrics and Gynecology, a US journal, with a second publication in the American Journal of Obstetrics and Gynecology this week. She said: ""While it is accepted that twin birth weights should ideally not be seen to differ greatly from each other, the degree of weight-difference which should be considered abnormal has always been a matter of debate. Our goal was to accurately define the difference in twin weights that translates into an increased risk of complications. The 18% cut-off point represents that threshold above which risk increases for both babies. We tend to think of the smaller twin as being the more vulnerable one, yet our study demonstrates that the larger twin of a discordant pair is equally at risk. Recognition of this threshold difference in weight during pregnancy should trigger closer fetal monitoring and in some circumstances may prompt early delivery."

The study also found that 77% of twin labour trials (natural labour, performed under observation by an obstetrician in order to achieve vaginal birth) were successful and infant morbidity rates were no different from those following a planned caesarean twin delivery.

While it is considered that the first twin must be ‘head-down' to allow a trial of labour, the research found that the position of the second twin prior to labour did not influence whether labour successfully resulted in a normal birth of both twins. In 4% of cases, the mother underwent a normal birth of the first twin, followed by caesarean delivery of the second baby; a rate somewhat lower than that observed internationally.

Dr Fionnuala Breathnach commented: "When it comes to how best to deliver twins, this is a contentious issue internationally. It is important to acknowledge that in more than half of twin pregnancies in this study, delivery was by caesarean section without any attempt at labour. Through this study we have demonstrated that under appropriate conditions and with careful patient selection, a high prospect of successful and safe vaginal delivery can be achieved with a trial of twin labour.

"Our study will enable obstetricians to take appropriate steps to minimize the risk during birth in order to provide the best outcome for mother and babies and enable healthcare professionals to support expectant mothers to make an informed choice in relation to the most appropriate birthing plan for their circumstances."

"Probably the most striking observation is the very low rate of serious infant morbidity and mortality among our study participants. Perinatal mortality in this Irish group of pregnancies was significantly lower than ever described before, particularly in the context of dichorionic (‘two-placenta') twins. The perinatal mortality rate among such twins was found to be lower than that described in most single birth groups. It is not entirely clear to what these impressive results should be attributed, but to me, it is in no small part due to the high standards of excellence in obstetric care for which Ireland has gained an international reputation," Dr Breathnach continued.

Enda Connolly, Chief Executive at the Health Research Board, said: "The research is of major significance both in Ireland and internationally. The findings will better enable healthcare professionals to make the optimum choices in the care of both the mother and offspring of twin pregnancies. It is vital that health research is directed at improving patient care and patient outcomes, and this is a great example of Irish research that will improve care in Ireland as well as having a major global impact."