The rate of twin birth is declining
Asma Khalil
Abstract
Most research papers and editorials on twin pregnancy published in the past 10 years began by acknowledging the global rise in the rate of twin pregnancy, primarily due to increasing maternal age and more widespread use of assisted conception. This trend is no longer the case. According to the 2018 Centers for Disease Control and Prevention (CDC) data in the USA, twin-birth rate declined by 4% between 2014 and 2018, reaching the lowest rate of 32.6 per 1000 total births in over a decade (Figure 1)1. This trend is mirrored in the UK: according to the 2018 Office for National Statistics (ONS) report, the rate of multiple birth has declined for the third consecutive year (Figure 1)2. Despite the similar trend, the USA has a higher rate of twin birth than the UK: for every 1000 women giving birth in the UK in 2018, 15 had a multiple pregnancy2. Since the first successful live birth following in-vitro fertilization (IVF) in 1978 and the subsequent rise in assisted conception, the rate of twin pregnancy has increased inexorably for more than three decades. Twin-birth rate began to rise in the USA in the early 1980s, increasing by approximately 80% from 1980 to 20143, 4. In 1980, one in every 53 births in the USA was a twin birth compared with one in every 29 births in 20143, 4. The largest increase in the multiple-birth rate was seen in women aged 45 years and over, with a rise from 15 multiple pregnancies per 1000 deliveries in that age group in 1978 to a peak of 115 per 1000 deliveries in 20122. This is likely to be due to the increased use of assisted conception. The most plausible explanation for the recent decline in twin births is the policy of single-embryo transfer and the improvement in IVF techniques, such as better freezing of extra embryos, genetic testing and culturing embryos until the blastocyst stage. These advances in IVF techniques have led to increasing prevalence of single-embryo transfer, as the chance of successful implantation has become higher. The fact that twin-birth rate declined among mothers aged 30 and over, with the largest decline among mothers aged 40 and over (23% decrease), supports this hypothesis. According to the USA statistics, there was no decline in twin-birth rate in mothers aged less than 30 years1. In the UK, the multiple-birth rate for women aged 45 years and over has fallen from 115.5 per 1000 women in 2012 to 79.3 per 1000 women in 20182. Of note, in the USA, the decline was seen in non-Hispanic white women, but not in Hispanic or non-Hispanic black women1. In January 2009, the UK Human Fertilisation and Embryology Authority (HFEA) introduced a policy to reduce the chances of multiple pregnancy following IVF treatment. This policy set an overall goal of reducing the national multiple-birth rate following conception by IVF to 10% and set a maximum multiple-birth rate that clinics must not exceed. National birth data, whether in the USA (CDC) or UK (ONS), are not reported according to the mode of conception. However, according to a HFEA report, the multiple-birth rate following IVF has decreased, as fertility clinics moved towards more single-embryo transfers. In 2017, the 10% multiple-birth rate target was achieved for the first time nationally, but not across all age groups. In 2018, the 10% target was reached across all age groups and, nationally, only 8% of IVF births resulted in a multiple birth5. It remains to be seen whether this trend will continue and whether it will plateau at some point. Despite the recent decline, the rate of twin birth is still higher than prior to 2008. More importantly, efforts should continue to improve the perinatal outcome of multiple pregnancies. The reported decrease in the UK national rate of stillbirth (nearly halved between 2014 and 2016, from 11.07 to 6.16 per 1000 total births) and neonatal death (one-third decline between 2014 and 2016, from 7.81 to 5.34 per 1000 live births) in twin pregnancies was welcomed6, 7. However, this trend was not sustained in 2017 (increase in stillbirths to 6.99 per 1000 total births and in neonatal deaths to 5.45 per 1000 live births), despite a decrease in stillbirth rate in singleton pregnancies8. Despite the fall in the rate of twin pregnancy over the past few years, it is essential to maintain focus on efforts to optimize outcomes for these higher-risk pregnancies.