From 2020 - 2023, Twins Trust partnered with St George’s Hospital, London, on the charity's first Centre for Research and Clinical Excellence. Through that partnership - and thanks to our ongoing work with hospitals throughout the UK - we've produced several pieces of research: 

Published research

Performance of Antenatal Diagnostic Criteria of Twin-Anemia-Polycythemia Sequence

Doctors don’t currently agree on the criteria they use to diagnose TAPS. There are different opinions about how to measure it, and at what point these measurements are significant enough to be diagnosed as TAPS. This means that there can be differences in how potential cases are diagnosed and managed, and how severe they appear to be, depending on the criteria used in each case. This study compares different sets of criteria to see how much they differ and is the first step towards finding a universal set of criteria which diagnoses cases appropriately. Because it is a rare condition, more research is needed across multiple hospitals to provide enough data for a definitive conclusion.

Perinatal outcomes in Pregnancies complicated by Spontaneous or Post-Laser Twin Anemia Polycythemia Sequence according to management option: Systematic Review and Meta-analysis

This study is an analysis of research that has already been carried out, to see if there are any common themes between different studies into twin anaemia polycythaemia sequence (TAPS). Researchers compared lots of different research papers’ findings about the effects of TAPS on the pregnancy, for spontaneous TAPS and TAPS which occurred after laser surgery. The study indicates that pregnancies with post-laser TAPS may be at higher risk of death, disability or preterm birth than pregnancies with spontaneous TAPS. This study didn’t show that a particular treatment has a better outcome. The study is limited because the research studies which were included are not exactly comparable. More research is needed to confirm the findings, and in the meantime doctors should assess each pregnancy with TAPS individually to decide on the treatment.

Twin chorionicity-specific population birth-weight charts developed with adjustment for estimated fetal weight

Following on from the pregnancy growth charts, there has been demand for a set of birthweight charts which show whether twins are within the expected weight range at birth, a priority highlighted by the Global Twins and Multiples Priority Setting Partnership. These charts have been developed based on 1,664 twin pregnancies, and show that twins are likely to be born smaller than singletons. They also highlight differences depending on the babies’ chorionicity (whether or not they shared a placenta during pregnancy). These charts will help to more accurately identify which babies, although smaller than singletons, are within the expected weight range for twins, and which babies have a growth problem which requires intervention.

Perinatal outcomes of small for gestational age in twin pregnancies: twin vs. singleton charts  

Twin growth is often assessed using singleton charts, leading to an overdiagnosis of babies who are deemed small for gestational age, which can lead to unnecessary intervention and early delivery. This study looked at the outcomes of 1740 twin pregnancies to see if twin charts were more effective at identifying twin pregnancies which were truly small and needing intervention. Twin charts identified a smaller number of the pregnancies as small for gestational age compared to singleton charts, but a higher proportion of those identified experienced neonatal complications. Babies identified as small by the twin charts were also more likely to have abnormal Doppler readings than babies identified as small by the singleton charts. This suggests that the twin charts may be better at identifying babies who are truly small and in need of treatment.

Adverse neonatal outcome in twin pregnancy complicated by small-for-gestational age: twin vs singleton reference charts

This study is a comparison of growth charts based on singleton and twin data, which estimate fetal weight during pregnancy and measure birthweight, in order to identify babies who are at risk of problems due to restricted growth. This study provides evidence that twin babies identified as small based on twin charts were more likely to have other problems in the neonatal period compared to twin babies identified as small based on singleton charts. Because of this, the paper concludes that twin charts are more accurate than singleton charts at identifying babies who need further support due to restricted growth.

Assessment of fetal growth in twins: which method to use?

This study looks at how best to measure growth in twin pregnancies. Evidence is limited, however this paper support previous findings that the association between babies identified as small and those who experience adverse outcomes, such as stillbirth, neonatal death and neonatal admission, is stronger when using twin-specific growth charts. This study recommends further research, such as a randomised trial, to validate the findings.

The association between hypertension in pregnancy and preterm birth with fetal growth restriction in singleton and twin pregnancy: Use of Twin versus Singleton Charts

This study was a comparison of singleton and twin pregnancy growth charts, to see how accurately they can assess growth in twin pregnancies, by looking at pregnancies with growth restriction. The twin charts showed a link between growth restriction, and hypertensive (high blood pressure) disorders and preterm birth, whereas the singleton charts did not show this link. This tells us that the twin charts more accurately identify the pregnancies with growth problems which may need intervention, compared with the singleton charts which may over-diagnose growth problems in pregnancies which don’t need any intervention. This study provides more evidence that to avoid over-diagnosing growth problems it may be better to use twin-specific charts to assess growth in twin pregnancies.

Are twin pregnancies complicated by weight discordance or fetal growth restriction at higher risk of preeclampsia?

This study examined the potential link between hypertensive (high blood pressure) disorders, such as pre-eclampsia, and growth restriction in twin pregnancy. The study concluded that there is a significant association between growth restriction and hypertension. This means that a pregnancy with at least one twin measuring small (compared to expected size for the number of weeks of pregnancy) is likely to experience hypertensive conditions like pre-eclampsia. For this reason, the study recommends that mothers’ blood pressure should be closely monitored if at least one baby is found to be small.

Maternal echocardiographic changes in twin pregnancies with and without pre-eclampsia  

This study has explored the impact of twin pregnancy on the mother’s cardiovascular (circulatory) system. The research showed that there were changes to the circulatory system both in pregnancies with a condition affecting the mother’s blood pressure, like pre-eclampsia, and in those without any blood pressure condition. When compared to pregnancies with only one baby, a twin pregnancy without any conditions affecting blood pressure showed the same changes as a singleton pregnancy with a blood pressure condition, which suggests that twin pregnancies put a high amount of strain on the mother’s circulatory system.

Non-invasive pre-natal screening in twin pregnancies with cell-free DNA using the IONA Test: a prospective multicentre study

Non-Invasive Prenatal Testing (NIPT) has been shown to be effective in singleton pregnancies, with high detection and low false positive rates for three chromosomal differences – trisomy 21 (Down’s syndrome), trisomy 18 (Edward’s syndrome) and trisomy 13 (Patau’s syndrome). There is potential for NIPT to be useful in twin pregnancies, as it may reduce the need for invasive testing which carries the risk of losing one or more babies. This study, which took place in six fetal medicine centres, evaluated whether the IONA test, a form of NIPT using a blood test taken from the mother, could accurately detect these three chromosomal differences. For Down’s syndrome (trisomy 21), this test was shown to be the most accurate screening test available in twins. It may be slightly less accurate for the other two conditions. The study recommends that the IONA test should be the primary test offered for Down’s syndrome screening in twins.

Key messages for obstetricians and fetal medicine specialists from the confidential enquiry into stillbirth and neonatal death in twins

This paper highlights the key findings and recommendations from the recent MBRRACE Confidential Inquiry into twin deaths, and is essential reading for healthcare professionals working in maternity care. The report found that in around 1 in 2 deaths care was poor, and better care may have prevented a baby dying. This paper identifies areas where care may have been poor (covering areas like preterm birth, antenatal care, TTTS, care in labour and more) and offers evidence-based suggestions on how these shortcomings can be addressed through practice and research. 

A call to action: Long-term neurodevelopment in twins is under reported, poorly understood and a growing public health concern. It is time to combine clinical excellence and academic rigour to improve outcomes that matter to families.

We know that twins, triplets and more show differences in their neurodevelopment compared to singletons. For example, they are at higher risk of cerebral palsy. Studies also show that monochorionic twins (who shared a placenta during pregnancy) are at higher risk than dichorionic twins (each with their own separate placenta). Currently, research is mainly focused on pregnancies with complications, such as TTTS. However, this leaves a large group of monochorionic twins who did not experience complications but do seem to be at higher risk of long-term effects, for which research is not currently being carried out. Additionally, the research into long-term effects of pregnancy complications is limited. The way that babies are followed up after birth varies between hospitals and this means that there isn’t enough comparable data to provide good evidence of the long-term effects these babies experience. This paper recommends that all monochorionic twins (whether from a complicated or uncomplicated pregnancy) are monitored into childhood for long-term effects, in a carefully planned way, so that we can see exactly what the prognosis is for monochorionic twins’ development. 

Controversies in the management of twin pregnancy  

This paper examines some of the key issues facing clinicians when caring for a multiple pregnancy, and the questions which remain unanswered around how best to provide this care. In particular, uncertainties around screening and management of specific pregnancy complications do not always have an obvious answer, and more evidence may be needed to resolve those issues. This paper outlines what these questions are and suggests further areas for research.  

Atypical twin-to-twin transfusion syndrome

This paper considers some cases where TTTS would be considered ‘atypical’, meaning it may not follow the usual trajectory and/or may need special treatment. Atypical cases include TTTS with sFGR (growth restriction); TTTS with heart problems but little difference in amniotic fluid levels, TTTS with TAPS that occurs before laser intervention, monoamniotic pregnancies with TTTS (where two babies share one amniotic sac), and cases where a gap appears in the membrane between the babies which can reduce the difference in fluid levels between the two sacs. The paper explains how to identify these cases and the evidence to date about treatments. This is vital information to be able to adapt treatment to the needs of individual pregnancies which have any of these factors. The paper recommends that doctors who treat these cases should combine their findings to provide the best evidence for these cases. 
Read more:

Ongoing research

The Registry, which was started in 2015, collects data on complicated multiple pregnancies, monochorionic twin pregnancies and uncomplicated triplet pregnancies.
Read more about how this data helps us in research.

Since 2017, ENCIRCLE has sought to understand if we can reduce or prevent preterm birth using emergency cerclage.
Read more about the ENCIRCLE study.

TWINS-RF aims to find out how complications in a twin pregnancy with a shared placenta can affect babies’ neurodevelopment at one year of age. Read more about the TWINS-RF study.

Monochorionic triplets, which share a placenta, are very rare, and can be at risk of complications such as Twin to Twin Transfusion Syndrome (TTTS) and having significantly different growth patterns, which in turn leads to a higher risk of stillbirth. We need to conduct research to find out the safest way of managing this kind of pregnancy.

Selective fetal growth restriction (sFGR) is a condition where one baby follows a typical growth pattern, while the other is much smaller than expected. This condition can affect both identical and non-identical babies. It carries a risk of stillbirth or very premature birth. More research is needed to make sure that doctors can differentiate between growth restriction which needs treatment, and a normal size difference between the babies, and to ensure they know how best to look after babies with this condition.

There is currently not enough evidence to indicate whether a twin pregnancy will be delivered successfully by vaginal birth rather than C-section. Understanding how we can predict a successful vaginal birth will allow doctors to give women more accurate advice when agreeing their birth plan.

Similarly, in some pregnancies where the first baby is delivered vaginally, complications can mean that the second baby needs to be delivered by C-section. Being able to identify risk factors for the second baby and understand why this happens could allow doctors to support parents better when deciding on the best way for them to give birth.

Obstetric cholestasis is a condition which can lead to extremely premature birth or stillbirth, and is more likely to affect pregnancies with more than one baby compared to singleton pregnancies. However, not much is known about the effect of the condition in twin pregnancies and more research is needed to understand how it impacts them and how doctors should manage these pregnancies.

We don’t currently know enough about the effect of gestational diabetes in twin pregnancies compared to singleton pregnancies. New research will try to establish who is likely to develop gestational diabetes and what the impact might be on the pregnancy and the babies.