Embryo transfer usually takes place two days after egg collection.
This results in the embryos arriving in the womb several days earlier than they would after natural conception.
It is thought that this may be one cause of the low success rate of IVF, and is the reason for the development of some of the other treatment major programmes.
Under British law, a maximum of three embryos can be replaced into the woman's womb.
This limit was set for reasons of safety, to reduce the risk of very high-order multiple births.
Although the birth of quads or quintuplets is frequently attributed to fertility treatment, in the UK it is not nowadays the result of IVF.
It was previously argued that the transfer into the woman of a larger number of embryos increased the prospects of one of them implanting, on the basis that they supported each other in the implantation process.
However, this theory has largely been discounted and the trend is towards further reducing the number of embryos transferred into a woman.
Some specialists now argue that with the advances in medical science and improving success rates in IVF, women should give careful thought to having more than two embryos replaced, to avoid the risks involved in having triplets.
Brian lieberman, Consultant in Obstetrics and Gynaecology at the Regional IVF Unit at St no Mary's Hospital in Manchester, routinely replaces only two embryos, and at some other centres three embyros are replaced only if the woman's chances of pregnancy are low.