Multiple births are a common occurrence in nature and happen as a result of one of two processes. Identical twins, triplets or more occur when a single fertilised egg splits to form more than one embryo. Non-identical multiple births occur when more than one egg is fertilised at the same time creating multiple embryos. The latter is more likely if a couple is having some form of infertility treatment.
Multiple identical births are two to five times more common after infertility treatment than in natural conception, although the reason for this is not fully understood. The high incidence of non-identical multiple births in couples who have had infertility treatment, on the other hand, is easily explained by multiple embryo transfers in which all the transferred embryos survive, or by the fact that fertility drugs stimulate the release of a group of eggs, which may then all be fertilised.
In the early days, infertility specialists viewed multiple births as an inevitable consequence of the procedure. However, as infertility treatments such as in vitro fertilisation (IVF) are now carried out so frequently, infertility clinics in many countries have begun to focus on the issue and create protocols to reduce multiple birth rates.
In the UK, for example, just 1.3 % of all births are IVF babies, yet 20 % of all multiple births are the result of in vitro fertilisation. The national body, the human fertilisation and embryology authority (HFEA), has now introduced regulations obliging all infertility clinics to have a multiple births minimisation strategy.
Other countries have gone further, legislating the number of embryos that may be transferred during IVF, often limiting this to a single embryo in women under 35 except in specific circumstances.
It might seem like the perfect idea to create your family in one go, with either twins or triplets, but the risks to both the mother and baby from multiple pregnancy and multiple birth are significant. A mother carrying more than one baby has an increased risk of:
Around 50 % of twins born after an infertility treatment such as in vitro fertilisation will need intensive care when they are born, and up to 10 % will need to spend more than four weeks in an intensive care baby unit. The risk of death in the early weeks is also increased by up to six times for twins and nine times for triplets.
Being the product of a multiple birth as a result of infertility treatment can also affect the children’s health for years to come, causing problems of lower IQ, behavioural issues and problems with language development, as well as an increased risk of diseases such as cerebral palsy and heart disease.
The best way to reduce the risks of multiple births is to transfer just one viable embryo at a time when a couple has IVF. This is called single embryo transfer, or SET. Other viable embryos can be frozen for use in later cycles should the first embryo fail to implant.
Clearly, the fewer embryos that are transferred, the lower the chances of a successful conception, but many studies have shown that overall birth rates hold up quite well when SET and frozen embryo cycles are combined. Sweden, for example, has a strict policy of SET, cutting their multiple births to just 5 %, yet still lies fourth in the world for live birth rates using in vitro fertilisation.
Single embryo transfer is a highly effective method for reducing the risks of multiple births for healthy women with a high chance of conceiving. However, if you are older, or have established problems that reduce your chances of successful conception from a single embryo, you may prefer to take the risk of having two or more embryos implanted in order to boost the odds.
One of the keys to successful single embryo transfer IVF is the ability to select the healthiest and most viable embryo for transplantation. This will be affected by the regulations that govern infertility treatment in your chosen country. For example, in the Czech Republic, embryos are allowed to develop for several days and be thoroughly tested before implantation. In other countries, they must be transferred much sooner and pre-implantation genetic screening (PGS) may not be possible.
State regulation varies widely, from a strict SET policy, through acceptance of two, three or four embryos as an upper limit, to countries such as the USA, Spain and Portugal where there are no legal limits. This situation led to the investigation of a US-based doctor who transferred 12 viable embryos into a woman who subsequently gave birth to octuplets in 2009.
SET is the norm in Northern Europe, with Sweden, Finland and Norway topping the ESHRE league. As with many other infertility treatments, Eastern Europe tends to take a more liberal attitude to embryo transfers, with Bulgaria, Hungary and Albania showing the lowest levels of SET.
In Italy, the Catholic laws insist that all fertilised embryos must be implanted, making the selection of the most viable embryos for SET impossible, and multiple births far more likely.