https://orcid.org/0000-0003-2802-7670
Sir,
We read with interest the recent study by Monden et al. (2021) reporting that the incidence of twins rose to an unprecedented level (from 9 per 1000 to 12 per 1000 deliveries between 1980–1985 and 2010–2015). We congratulate the authors for drawing together global data on the frequency of twin births, as well as studying the variation among countries since the 1980s. Of note, this study included data only up to 2015. We would like to highlight the decline in twin births reported since 2014, both in the UK and in the USA. In the UK, there has been a 4.4% decline between 2014 and 2019, from 16.0 per 1000 to 15.3 per 1000 maternities ONS data. This pattern is mirrored in the USA, where a 5.3% decline, from 33.9 to 32.1 per 1000 total births was reported during the same period (CDC data) (Fig. 1).
Rate of twin births in the USA and UK, 1982–2019.
The explanations for the rise in the incidence in twins until 2015 reported by Monden et al. (2021) are increasing maternal age (associated with a higher incidence of twins) and more widespread use of medically assisted conception. One plausible explanation for the reverse in this trend seen in the UK and USA since 2014 is the policy of single embryo transfer and the improvement in IVF techniques. This hypothesis is supported by the fact that the decline in twins was seen among mothers aged 30 and over, with the largest decline among mothers aged 40 and over. The twin birth rate declined from 66.0 to 51.1 per 1000 total births between 2014 and 2018 (a 23% decrease) (CDC data). A similar trend was seen in the UK, where in women aged 45 years and over, the multiple maternity rate has fallen from 115.5 per 1000 women in 2012 to 79.3 per 1000 women in 2018 (ONS data).
The UK Human Fertilisation and Embryology Authority (HFEA) introduced a single embryo transfer policy in 2009 in order to reduce the chances of multiple pregnancy following IVF treatment. They set an overall national goal of reducing the rate of multiple births conceived via IVF to 10%. This national target was achieved for the first time in 2017.
It seems that this decline in twin births is continuing and it is unlikely that the data for 2020, dominated as they are by the unprecedented COVID-19 pandemic, will show a reverse in this pattern. Services in fertility clinics were disrupted during the pandemic resulting in a significant drop in the number of IVF cycles in 2020. It is likely, but remains to be demonstrated, whether this has resulted in a significant fall in the number of IVF pregnancies, in particular those resulting in twin or multiple births, in the past year. Moreover, it is yet to be ascertained whether this decline observed in both the UK and the USA is also seen globally, including low- and middle-income countries.
An important question is whether this recent decline in twin births is associated with improved pregnancy outcomes, as might be expected, given the increased risk of adverse outcomes associated with twin pregnancies. In the UK between 2014 and 2016, a decrease in stillbirths (nearly halved from 11.07 to 6.16 per 1000 total births) and neonatal deaths (one-third decline from 7.81 to 5.34 per 1000 live births) in twins was reported (Draper et al., 2018). However, in 2017, both stillbirths and neonatal deaths increased to 6.99 per 1000 total births and 5.45 per 1000 live births, respectively, despite a fall in the stillbirth rate in singleton pregnancies (Draper et al., 2019). In 2019, the stillbirth rate in twin pregnancies remains twice that in singletons, and continues to increase despite a concomitant decline in singletons (ONS data). Sadly, according to the most recent report by Reducing Risk through Audits and Confidential Enquiries across the UK (MBRRACE), ‘in around one in two baby deaths in twins, the care was poor, and that if the care had been better it may have prevented the baby from dying’ (Draper et al., 2020). Therefore, it is important to urge the governments to invest more healthcare resources and staff training to improve the outcome of these high‐risk pregnancies.
Editorial by-line: The authors of the original published paper, Monden et al., 2021, were offered the opportunity to publish a formal response to this Letter, but declined to do so.
Conflict of interest
None.
References
https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/livebirths/datasets/birthcharacteristicsinenglandandwales (13 March 2021, date last accessed).
https://www.cdc.gov/nchs/fastats/multiple.htm (13 March 2021, date last accessed).
https://www.cdc.gov/nchs/products/databriefs/db351.htm (13 March 2021, date last accessed).
