Ms R. Simmons, Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
The proportion of multiple births has increased markedly since 1980 in England and Wales. A major contribution to this trend is thought to be the widespread introduction of assisted reproductive technologies. Despite a continuing (but slower) upward trend in twin maternities, analysis of recent data shows that the rate of triplet and higher order births in England and Wales has declined by one-quarter since 1998. This probably reflects both voluntary and statutory regulation of treatment regimes. This downward trend will help alleviate the high burden of perinatal morbidity and mortality associated with multiple births.
Rates of all multiple births in England and Wales have increased by more than 50% in the past two decades. Much of this upsurge has been attributed to the use of assisted reproductive technologies,1 and there has been continuing debate over the optimal number of embryos to replace after in vitro fertilisation (IVF) treatment and the number of eggs to transfer in gamete intra-fallopian transfer. Clinicians have tended to optimise the chance of successful implantation by increasing the number they replace, but the resulting rise in multiple births has been met with concern by the medical community. In a bid “to prevent triplet gestations and to reduce the number of twin gestations”,2 recent Human Fertilisation and Embryology Authority (HFEA) legislation further limited the number of eggs or embryos that can be transferred during a single cycle of infertility treatment to a maximum of two for women under 40 years (three for women 40 years and over). We report here on recent trends in the number of triplet and higher order births in England and Wales.
Methods and results
We looked at national birth trends in England and Wales using data provided by the Office for National Statistics. We found that multiple births continue to account for an increasing percentage of all births (Table 1). In 1980, for example, the proportion of multiple maternities was 1.0%. By 2001, this figure had risen to 1.5% (i.e. by 51%). However, a closer examination of these figures shows that rates of twin and triplet plus higher order births are diverging. Since 1998, twin maternity rates have continued to increase, albeit at a slower rate (Fig. 1), but there has been a dramatic decrease in the proportion of triplet and higher order births over the same period (Fig. 2). This decrease was seen in all women aged 25 years or more, with rates remaining fairly constant among mothers younger than this. In addition, women in the oldest age groups (≥30 years) experienced a sharp decline in triplet and higher order births from as early as 1992 (Fig. 2), perhaps following HFEA legislation limiting the number of embryos to be replaced after IVF treatment to three.
Table 1. Numbers and rates (%) of all multiple, twin, triplet and higher order maternities 1980–2001 (England and Wales).
Overall, these data suggest that, from 1997, clinicians and patients responded to increasing concern about multiple births by changing treatment policies, for example, by choosing to replace less than three embryos following an IVF cycle. There may have also been a corresponding reduction in the number of eggs replaced in gamete intra-fallopian transfer cycles or in cycles using superovulatory drugs, which are not regulated under HFEA. Thus, this dramatic reduction in the number of triplet and higher order babies born in England and Wales is probably associated with both voluntary and statutory regulation. This reduction will help improve perinatal outcomes and maternal health,3 as well as reducing the financial burden of such events.
Discussion and conclusion
While the proportion of triplet and higher order births has reduced, these figures have had little impact on the proportion of multiple births overall, which are dominated by twin maternities. It is therefore important to continue to monitor trends in multiple births in England and Wales. Despite the decline in the rate of increase of twin maternities, rates for women over 25 continue to rise, particularly among women aged 30–34. Assisted conception techniques are likely to improve over time and this will further increase the probability of successful implantation of two embryos following IVF. The proportion of multiple maternities is therefore unlikely to decrease unless the use of single high-quality embryo transfers in IVF is promoted,4 especially for women aged under 35, and this may require further legislative change by the HFEA.