IVF - the risks

Although in vitro fertilisation (IVF) births account for only a small percentage (1.3%) of all births nationally, over 20% of all multiple births are IVF births. This means that IVF births contribute a disproportionately large number of multiple births to the overall rate.

At present, about 1 in 4 IVF pregnancies result in multiples: this is around 20 times higher than the rate after natural conception.

How does IVF work?

During IVF, eggs are removed from the ovaries and fertilised with sperm in a dish in the laboratory. The developing embryos can be left for either 2 to 3 days, or 5 to 6 days before being transferred to the woman’s womb.

• After 2 to 3 days, the embryos are known as ‘cleavage stage’ embryos. See Embryo transfer: 2 - 3 days for more information.
• If the eggs are left to develop for 5 to 6 days after fertilisation, the developing embryos are known as ‘blastocysts’ and the procedure of transferring them to the womb is known as ‘blastocyst transfer’. See Embryo transfer 5 - 6 days for more information.

How does IVF affect the risk of multiple birth?

Multiple birth is the single biggest risk to the health and welfare of children born after IVF. See Risks to the mother and Risks to the child.

Currently, after IVF and intra-cytoplasmic sperm injection (ICSI) around 24% of pregnancies are multiple, compared with only 1 – 2% after natural conception.

This can be directly related to the fact that IVF normally involves the transferring of 2 or 3 embryos to the womb.

• At present, according to HFEA guidance, the number of embryos that can be transferred is no more than 2 for women under 40 (or for those receiving donated eggs as donors are under 35) or no more than 3 for women over 40.
• If two separate embryos implant in the womb, the mother will be pregnant with non-identical twins.
• The chance of a single embryo dividing and resulting in identical twins is also higher after IVF though it is not yet known why this happens. So it is possible to end up with twins from a single transferred embryo, or triplets from 2 embryos.
• Most of the triplets born after IVF are the result of double embryo transfer.
• Data from the UK (2005) shows that almost 46% of babies born as a result of IVF to women under 35 (using fresh eggs) are multiple births. 

What are my options?

If you or your clinician considers that you are at risk of multiple pregnancy, you should discuss transferring only 1 embryo (single embryo transfer, known as SET).

• If only 1 embryo is transferred during IVF (single embryo transfer), the risks of multiple birth are greatly reduced (although the risks are not completely eliminated as you are still 2 to 5 times more likely to become pregnant with identical twins after receiving fertility treatment than after natural conception). 
• Your clinic will discuss whether this is suitable for you. If it is, they may suggest transferring a single embryo and freezing any remaining ones for future use.
• The latest research shows that, if you are young, healthy and have a good chance of conceiving with IVF, transferring only one embryo does not significantly reduce the chance of pregnancy.

For further information see

• Chance of success with SET
• What is IVF - How does it work (HFEA website)

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