IVF - the risks

Newborn baby in hospital

About one in six IVF pregnancies result in a multiple birth. This is very high compared with natural conceptions of twins which occur in about 1 in 80 births.

Multiple births following fertility treatment present significant health risks to mothers and babies. See Risks to the mother and Risks to the child.

With IVF these risks are avoidable with the use of elective single embryo transfer (eSET) and frozen embryo transfer in appropriate patients.  

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 two to three days, or five to six days before being transferred to the woman’s womb.

  • After two to three days, the embryos are known as ‘cleavage stage’ embryos.
  • If the eggs are left to develop for five to six 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 grading and freezing for more information.

How does IVF affect the risk of multiple births?

Currently, after IVF and intra-cytoplasmic sperm injection (ICSI) around 16% of pregnancies are multiples, compared with only 1–2% after natural conception.

This can be directly related to more than one embryo being transferred during IVF and ICSI.

  • 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, particularly after blastocyst transfer, however it is not yet known why this happens. So it is possible to end up with twins from the transfer of a single embryo, or triplets from two embryos.
  • Data from the UK (2013) shows that almost 15% of babies born as a result of IVF to women under 35 (using fresh eggs) are multiple births.
  • Although HFEA guidance states that the number of embryos that can be transferred is no more than two for women under 40 (or for those receiving donated eggs as donors are under 35) or no more than three for women over 40, the aim for all patients should be to have a single healthy birth.
  • Because of this all clinics must now have a strategy to minimise the risk of a multiple pregnancy, by using elective single embryo transfer where appropriate.

What are my options?

Your clinician will discuss your risk of having a multiple pregnancy and will recommend single embryo transfer if appropriate. Your clinic will discuss the best option for you.

  • If one embryo is transferred during IVF (single embryo transfer), the risks of multiple birth are greatly reduced.
  • Where additional good quality embryos are available, the clinic will advise freezing these for future use.
  • In some cases, the clinic may suggest transferring two embryos depending on your individual circumstances.
  • 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 reduce the chance of pregnancy.

For further information see


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Multiple birth factsheet for patients

Are you going through fertility treatment and want to know more about the risks of multiple births?

Infertility Network UK (INUK) has published a factsheet that addresses a number of common concerns and questions that fertility treatment patients have.

Have you had or begun IVF treatment recently?

The HFEA want to hear your views on single embryo transfer.

Your feedback will help them develop their approach to the risk of multiple births from fertility treatment.

  • What did your clinic discuss with you?
  • How did you decide on the number of embryos to transfer?

Let them know in this patient questionnaire