Pregnancy and child

Single embryo IVF best for healthier baby

“Women who have one embryo transferred during IVF treatment are five times more likely to give birth to a healthy baby,” reported The Guardian . It said that “those who have two embryos are more likely to get pregnant but are at a greater risk of delivering a premature or low-weight child”.

This news report is based on a systematic review and meta-analysis of trials comparing in vitro fertilisation (IVF) in which one embryo is transferred, to that in which two embryos are transferred. As The Guardian reports, single embryo transfers were associated with a lower risk of having a preterm, low birthweight child compared with double embryo transfers.

This is well-conducted research and its findings support the current recommendations that the number of embryos transferred during IVF should be limited to avoid the associated risks of multiple births. More information can be obtained from the oneatatime website, which is aimed at better outcomes for fertility treatment.

Where did the story come from?

The study was carried out by researchers from the University of Aberdeen and other institutions worldwide. The research was funded by the Wellcome Trust. The study was published in the peer-reviewed British Medical Journal.

The Guardian ’s report of this study was accurate. The Daily Express said that having two rounds of IVF using one embryo each time is better than having two embryos implanted during one round because transferring more embryos leads to a higher risk of miscarriage. The study, however, found that single embryo transfer was associated with higher rates of miscarriage than double transfer, but highlighted that this was uncertain as there was variation between the trials. The newspaper also said that women who had a fresh embryo implanted and then in a later attempt had a frozen embryo implanted ended up with more children. The study found that women who had this sequence of procedures had a similar cumulative number of live births to women who had double embryo transfer. This means that women get two chances at IVF without the risks of double transfer.

What kind of research was this?

This research compared the number of miscarriages, live births and baby weight between single embryo transfer IVF and double embryo transfer. The researchers were also interested in whether there was a difference in these outcomes if the embryo had been frozen before transfer.

IVF involves surgically removing an egg from the woman's ovaries and fertilising it with sperm in a laboratory. The fertilised egg, or embryo, is then placed back into the woman's womb to grow and develop.

There is an increased chance of producing twins or triplets with IVF treatment if more than one embryo is put back into the womb. This can be problematic, as having more than one baby in the womb significantly increases the risk of complications for the mother and babies. Around half of all twins and 90% of triplets are born prematurely or with a low birthweight. The risk of dying in the first week of life is also greater for twins and triplets.

This was a systematic review and meta-analysis of previous research, comparing single with double embryo transfers during IVF.

What did the research involve?

The researchers selected randomised controlled trials that had compared single with double embryo transfers during IVF. These trials were in women who had standard IVF or intracytoplasmic sperm injection (where the sperm is injected directly into an egg to fertilise it). The women were using their own eggs for these treatments rather than using donated eggs. Trials were included if they used the same IVF methods, differing only in the number of embryos transferred.

In total, the researchers used data from eight studies, which included 1,367 women. Six hundred and eighty women had single embryo transfers and 684 had received double embryo transfers.

What were the basic results?

IVF using single embryos resulted in 27% live births compared with 42% of those using double embryos. This was found after factors that may affect the outcome of IVF were taken into account, including the duration of fertility problems, the main cause of infertility (a male factor, a female factor, both or unexplained), the type of IVF, the woman’s age and BMI, the number of embryos available for transfer and the quality of embryos transferred.

The researchers found that the chance of having a live birth with single embryo transfer was half that of double embryo transfer (adjusted odds ratio, 0.5, 95% confidence interval [CI] 039 to 0.63).

Of women who had received single embryos, 2% had multiple births compared with 29% of women who received double embryos.

Overall, the live birth rate using fresh embryos was lower after single than double embryo transfer.

The researchers then looked at whether there was a difference in the birthweight or the number of babies born prematurely following single or double embryo transfer.

The chance of delivering at least one low-birthweight baby (under 2.5kg) after single embryo transfer were a third of that from double embryo transfer. The average weight of babies delivered was similar in single and double embryo transfer, at 3,373g and 3,275g, respectively.

The chance of having a full-term singleton birth (over 37 weeks) was about five times higher following single embryo transfer compared with double embryo transfer (odds ratio 4.93, 95% CI 2.8 to 8.18).

The researchers also looked at two of trials that assessed additional frozen single embryo transfer after the initial fresh elective single embryo transfer. The cumulative live birthrate of this strategy (38%) was similar to that after one fresh double embryo transfer (42%). This means that this sequence may put the chance of having a baby following IVF to a similar level to transferring two embryos in the same IVF round without the risks associated with double embryo transfer.

How did the researchers interpret the results?

The researchers said that single embryo transfer results in increased rates of full-term live births (compared with double embryo transfer during IVF) and decreases the risk of having a premature baby or a low birthweight baby.


This was a well-conducted systematic review, demonstrating some advantages of single embryo transfer during IVF for having a baby, despite a lower overall birthrate compared with transferring two embryos. There are a few limitations in how these findings can be interpreted:

  • The researchers point out that nearly all of the trials studied in this review were in women with a good prognosis for having a successful IVF outcome (younger women with good quality embryos). As such, it may not be possible to generalise these findings to older women or women with poorer quality embryos.
  • Most of the trials had looked at single or double embryo transfer of embryos that had not been frozen. Only two of the eight studies looked at the cumulative outcomes of fresh followed by frozen single embryo transfer. Further research is needed to see how this would affect cumulative birth rate in a larger population and to see whether this strategy would result in a birth rate similar to that resulting from double embryo transfers.

This study suggests that although single embryo transfer is associated with a lower birth rate than double, it has a reduced risk of premature birth and low birthweight babies. The higher risk of premature and low birthweight babies with double embryo transfers may be due to the increased likelihood of having twins. This supports the current recommendations that the number of embryos transferred during IVF should be limited to avoid the associated risks of multiple births.

NHS Attribution