Main findings

We have shown a continued trend of increasing multiple pregnancies in England, in terms of rates and absolute numbers of deliveries. The findings highlight that this is due to an increase in the rate of twin maternities over the past 12 years. In contrast, the rate of triplets and higher order maternities has halved. Substantial differences were seen by socioeconomic deprivation with a lower rate of multiple births among women from the most deprived areas and this was most evident among women over 35 years of age. Stillbirth rates have fallen considerably among twin births unlike the static picture seen for singletons. The recent improvements also observed in neonatal mortality have benefited singleton and twin births, but wide socioeconomic inequalities exist in mortality for all births.

Possible explanations for findings and comparisons with other work

The observed reduction in the rate of triplet and higher order births continues the pattern observed by Simmons et al6 up until 2001, and coincides with changes in the regulatory framework in England governing ART. Although the chances of a successful implantation when undergoing fertility treatments such as in vitro fertilisation (IVF) or gamete intrafallopian transfer treatment is significantly improved by increasing the number of embryos or eggs transferred, multiple births, particularly triplets and higher order pregnancies, are at significantly greater risk of poor outcome compared with singletons. Therefore, in 2001, the Human Fertilisation and Embryology Authority (http://www.hfea.gov.uk/) introduced regulations to limit the transfer of a maximum of two embryos per cycle, except in exceptional circumstances. With sporadic compliance, this policy was tightened further in 2004, so that a maximum of two embryos could be transferred to women under the age of 40 with no exceptions, and a maximum of three transferable to women aged 40 and over. Further evidence suggests that in women under 37 years, elective single embryo transfer is recommended14 to improve outcomes. Currently UK fertility units must have a maximum multiple birth rate of 10% (http://www.hfea.gov.uk/). Our findings of a reduction in the rate of triplet and higher order maternities coincide with these changes in regulation. A possible additional factor influencing the rate of triplets could be the increased frequency of fetal reduction in multifetal pregnancies in the first trimester. In the UK, death of an unborn fetus would not be registered if delivered before 24 weeks of gestation. For those deliveries where a fetal reduction occurred before 24 weeks of gestation and the fetus was known to have died, the death should not be registered as a stillbirth but there may be some variation in interpretation of the legislation.

Our data suggest that women from deprived areas, particularly those over 35 years, were less likely to have a multiple birth than women from less deprived areas. While our data cannot determine the reasons for this, one possible explanation is differential access to cycles of assisted reproductive techniques. Carson et al15 using data from the UK Millennium cohort showed that the income of families of infants conceived through ART was substantially higher than for families of infants resulting from planned or unplanned natural conceptions. A UK survey of PCTs has indicated that in the vast majority of trusts, there is provision for only one cycle of treatment paid for by the National Health Service (NHS) (http://www.dh.gov.uk/en/Publicationsandstatistics/Publications/PublicationsPolicyAndGuidance/DH_101073 2009). Consequently, further cycles of treatment need to be paid for by the couple and with charges between £4000 and £8000 per cycle, this is likely to exclude low income couples. National Institute for Health and Care Excellence (NICE) guidelines have recently been updated from those written in 2004 and now recommend up to three cycles of IVF for women under 40 years and one cycle for women aged 40–42 years16 being paid for by the NHS. For those areas under the new commissioning architecture that follow these guidelines, inequalities in access may be reduced with a consequent increase in multiple maternities in the most deprived women. No national data exist to explore access to assisted conception by deprivation and research is needed to assess this issue. Similar socioeconomic inequalities in multiple birth rates are likely to be seen in developed countries with similar provision of ART, but the deprivation gap is likely to be even greater in those countries where ART is only available privately.

We have observed considerable reductions in stillbirth rates over time for multiple births unlike singletons, where rates have remained static for the last decade. This may relate to the introduction of recommendations for changes in antenatal care, including improved early diagnosis and in utero management of twin–twin transfusion syndrome in monochorionic twins. However, Glinianaia et al17 in a smaller UK regional study found no change in stillbirth rates over time for either monochorionic or dichorionic twins, but monochorionic twins have a considerably higher risk of stillbirth than dichorionic twins predominantly due to twin–twin transfusion syndrome. The apparent improvement in stillbirth rates we have observed may also be partially explained by a change to the proportion of monochorionic versus dichorionic twins over time. Around 16% of assisted conception multiple pregnancies result in monochorionic twins,18 and so a rise in the proportion of multiple births arising from ART would lead to a substantial rise in the proportion of dichorionic twin births. If this is the explanation, it should then be possible to observe an overall reduction in the rate of stillbirth for twins but no improvement in the chorionic specific rates of stillbirth. Since there are no national data on chorionicity, we cannot determine to what extent the changes in stillbirth rates are related to changes in the proportion of monochorionic twins or to actual improvements in care. However, stillbirth rates for twin births improved across all deprivation groups and so it is likely that multiple factors contributed to the observed change in stillbirth rates.

Compared to singletons, there was a smaller deprivation gap in the rate of stillbirth and neonatal death for twins and no significant deprivation gap for triplets and higher order births. However, the absolute deprivation gap was wider. Research on explanations for the deprivation gap in neonatal mortality among singleton births has shown that it is predominantly explained by increased rates of prematurity and its associated complications.19 Multiple births are at much higher risk of prematurity with the rate of preterm birth (<37 weeks gestation) being 10 times higher among multiple births and with half of all multiple births being born at these gestations.11 Intrauterine growth restriction is also associated with deprivation20 and increased complications of prematurity. However, it is suggested that small-size and premature delivery in the case of multiple births may be more related to physiological adaptation to the more limited intrauterine environment21 in contrast to the proposed mechanisms in singleton births where maternal or fetal pathology is often implicated. Without further research, it is not possible to estimate the extent to which prematurity is responsible for the deprivation gap in neonatal deaths among multiple births.

Strengths and limitations

This study uses national routinely collected data to evaluate trends in multiple birth rates. These data have the advantage of being readily available and having national coverage. They lack detail on the chorionicity of the multiple births and so differences in trends could not be investigated between monochorionic and dichorionic twins. While the focus of increased twinning has been associated with dichorionic twins relating to assisted conception techniques, data are now indicating that these techniques also lead to increases in monochorionic twins and outcomes in these twins are particularly poor.17 We also did not have information on the use of ART. While information on chorionicity and the use of ART is not currently available nationally, data collection on these factors will start from January 2013 as part of the MBRRACE-UK programme. This will provide a national picture of chorionicity and ART among stillbirths and infant deaths in the future.

The data we obtained did not link births from the same maternity. Consequently, we had to estimate the number of multiple maternities which is prone to error as a result of a lack of data on late fetal losses. Similarly, early selective feticide for congenital anomalies may lead to misclassification of some multiple pregnancies as singleton births. However, our sensitivity analyses comparing the overall estimated number of multiple maternities with published data showed that this method estimated the overall number to within 1% of the actual rates and so any impact on the findings presented here is likely to be small. Unfortunately, in the analysis of stillbirths and neonatal deaths, we could not take account of the correlated nature of the outcomes of multiple births from the same pregnancy. The CIs presented here are consequently likely to have been narrower had this adjustment been possible, but this is unlikely to have impacted on our conclusions. National data were also unavailable on gestational age for this time period which prevented exploration or adjustment of mortality for prematurity. Furthermore, while national published data are available, they do not offer the ability to explore interactions between risk factors.

Implications and future research needs

The continuing rise in multiple birth rates and overall increases in births over the past 12 years has had a large impact on the absolute numbers of twin and triplets delivered in England. While there has been a reduction in the rate of triplet and higher order births, which are the most at risk of neonatal death, there are now around 1300 more multiple births a year in England compared with 1997. Since over half of all multiple births are born prematurely,11 increasing healthcare provision and NHS costs for neonatal and longer term care arising from this group of babies will ensue. These high risks for multiple births support the policies in place to reduce the rate of multiple births in the UK and the need to consider reviewing the practice of allowing multiple egg or embryo transfer in older women.

Current national data prevent detailed exploration of socioeconomic inequalities in access to ART in England. However, recently established procedures for national data collection of this information will enable monitoring of such trends in the future. This will also permit an assessment of whether recent changes in the guidelines for provision of assisted conception techniques in the UK, increasing the permitted number of cycles open to women under 39 years, have led to improved access to ART services across all socioeconomic groups.