Figures released by the UK's Human Fertilisation and Embryology Authority (HFEA) reveal the number of IVF cycles performed each year has continued to rise while the overall multiple pregnancy and birth rate has declined.
Meanwhile, figures from the Office for National Statistics (ONS) show that the overall number of babies born in England and Wales has increased by more than a fifth in the past decade and the multiple birth rate in the general population has increased.
The HFEA reports that in 2011 48,147 women received 61,726 cycles of IVF or ICSI, around four percent more cycles than in 2010. The overall live birth rate per cycle has remained steady at around 24 percent between 2009 and 2010, with a long term increase from 14 percent in 1991 to 24.5 percent in 2010.
It also said that in 2011 six out of ten IVF cycles were funded privately, with no perceived change since 2010. Around three percent of women who had IVF or ICSI did so as part of an egg sharing agreement or to produce eggs or embryos for donation.
The ONS reports that with the exception of a small fall in 2009, births have been increasing every year since 2001. There were 723,913 live births in 2011, rising from 594,634 in 2001. According to the ONS, just over 16 in every 1,000 deliveries in 2011 were multiple births, compared to a rate of 14.8 a decade earlier.
Women aged 45 and over had the highest multiple pregnancy rate with an almost one in ten chance of giving birth to twins or triplets. In 2011, 11,330 women gave birth to twins and just three to quads or more.
The HFEA figures show that the overall multiple pregnancy rate following fertility treatment fell from 26.6 percent in 2008 to 20.1 percent in 2011, however. The greatest decrease was seen in women aged between 18 and 34 who accounted for the greatest increase in elective single embryo transfer.
Fertility treatment is far more likely to result in multiple births than natural conception. On average, around a quarter of IVF pregnancies result in multiple births compared to one percent among women who conceive naturally. Around 1.5 percent of all births in the UK each year are from IVF or ICSI.
As part of a national strategy to reduce the multiple birth rate, the HFEA adopted a policy to allow clinics to establish their own strategies to promote single embryo transfer for those considered to have the highest chances of conceiving, even when more are available. The HFEA also sets a maximum multiple birth rate for clinics, currently set at ten percent.
Explaining the rise in multiple births in the ONS figures, Dr Yacoub Khalaf, a consultant gynaecologist at the Assisted Conception Unit of Guy's and St Thomas' Hospital in London, told the Telegraph: 'It has long been recognised that most multiple pregnancies are from non-IVF treatment, which is mostly the use of stimulation drugs'.
'GPs and hospital consultants are prescribing these drugs and they can be quite generous. Doctors in the community can just give the drugs and hope for the best. If the process is not monitored rigorously enough you can end up with what we are observing here with these figures', Dr Khalaf explained.
He also said that increasing numbers of older mothers conceiving naturally and couples travelling abroad to receive unregulated fertility treatment may account for the rise in multiple pregnancies.
For women, multiple pregnancy carries an increased risk of miscarriage and pre-eclampsia, among other complications. In addition, half of all twins are born prematurely and are more likely to have cerebral palsy than single babies.
The HFEA collects data from clinics in England, Wales, Scotland and Northern Ireland. The ONS data is compiled for births in England and Wales only and will include births in these regions from fertility treatment presented by the HFEA.