Leaving the embryo to develop into a blastocyst before transfer to the womb can increase its chances of successful implantation, says Dr Gillian Lockwood

What is a blastocyst?
When a sperm fertilises an egg, the resulting embryo develops from the original single cell into a complete human being, consisting of billions of cells. This occurs through a series of cell divisions into two cells, then four, then eight and so on.

By about four days after fertilisation, there is a ball of cells, which then absorbs fluid and expands to form a cavity. The embryo is now called a blastocyst and consists of a layer of outer cells that eventually become the placenta, and a small number of inner cells, which will form the foetus.

At this stage, about a week after fertilisation, the embryo has completed its journey down the Fallopian tube and reached the uterus. About this time the blastocyst hatches out of its thin shell (the zona pellucida) and implants in the womb lining.

What is blastocyst transfer?
When a woman has conventional IVF or ICSI, she usually returns to the fertility unit to have the embryos transferred two or three days after egg collection. But with blastocyst transfer, the embryos are cultured for a further two days in a special culture medium that mimics the nourishing fluids found in the Fallopian tubes, by which time they should have developed into blastocysts.

What are the advantages?
IVF units around the world have seen an increase in the clinical pregnancy rate per embryo transfer when blastocysts are transferred instead of four or eight cell embryos. Not only are blastocysts more likely to implant and grow, but fewer need to be transferred and so the multiple pregnancy rate is lower.

Why might the pregnancy rate increase?
There are two theories to explain why blastocyst transfers might result in a higher pregnancy rate. Not all embryos have the potential to produce a full term pregnancy. By allowing embryos to grow in a highly developed culture medium, it is possible to identify those embryos that are developing faster and are more likely to result in a pregnancy.

During natural conception, the embryo does not normally reach the uterus until day four or five. By delaying transfer, the embryos can be placed in the uterus at the same stage that would occur naturally.

Who would blastocyst transfer be suitable for?
Women who have had multiple embryo transfers and not achieved a pregnancy may benefit from blastocyst transfer, as this technique allows the most vigorous embryos to be identified and transferred.

Are there any risks?
Pregnancies that have resulted from blastocyst transfers seem to show an increase in the incidence of identical twins. There do not seem to be any other risks, but this is still a new technique and further research is needed.