IVF babies born from frozen embryos are heavier and healthier than those born from fresh embryos, new fertility research has discovered.

The research by the Centre for Reproductive and Genetic Health found that frozen embryos are bigger and have better health as a result of longer pregnancies, giving the foetus more time to grow into a healthy birth weight than fresh embryos, which were born on average 0.65 weeks earlier.

The findings, presented at the British Fertility Society Annual Meeting in Leeds, involved measuring the weight and length of gestation of 384 single babies born after a fresh embryo transfer and 108 born from frozen embryos.

Researchers discovered that babies born from a frozen embryo through IVF were on average, 253g heavier than those born from fresh. They also found that the proportion of low birth weight babies (weighing less than 2.5g) was also lower in the frozen group (3.7% compared to 10.7% for babies born from fresh embryos).

“For all assisted reproduction technologies, it is important that we ensure the procedures promote optimal health in the resulting children throughout their lives,” says lead researcher, Suzanne Cawood, deputy head of embryology at the Centre for Reproductive and Genetic Health.

“Our study suggests that babies born from frozen embryos have a significantly longer gestation period and are significantly heavier at birth compared to babies from fresh embryos.

“This is important because prematurity and low birth weight are both risk factors for poorer health later in life and are linked to higher rates of behavioural and learning difficulties.

“This means that resulting babies may potentially be healthier if frozen embryos are transferred rather than fresh embryos.

“The reasons behind these findings are not yet fully understood, but one possibility may be that there is a difference in the uterine environment between fresh cycles, when embryos are transferred soon after the eggs have been collected, compared to frozen cycles when the uterus has not been stimulated in the days before transfer.

“However, further research is needed to test this hypothesis,” Cawood added.

The recent findings follow a Danish study in 2008, which found that babies born from a frozen embryo had higher birth weights than those born from fresh embryos. Danish researchers suggested that this was because only top quality embryos survive the cryopreservation process of being frozen as well as the thawing process before IVF.

Researchers hope that these positive findings will cut the number of multiple pregnancies from IVF as it supports the move of transferring a single embryo at a time.

“These initial findings, if proved accurate following further research, will give the medical profession more evidence to encourage patients to accept single embryo transfer, which reduces the risks of multiple births to both mother and babies,” says Clare Lewis-Jones, chief executive of the Infertility Network.

“Single embryo transfer gives the best possible outcome - a healthy singleton baby - with the chance of further frozen embryo transfers in the future. If these results prove positive, then we would welcome this and hope it benefits infertile couples everywhere.”

In the UK, over 45,264 women have IVF treatment, according to the Human Fertilisation & Embryo Authority (HFEA). These women go on to have 57,652 cycles between them resulting in 12,714 babies being born.

“If IVF babies born from a frozen embryo are heavier and pregnancies last longer, communication between the mother and midwife is key,” says independent antenatal teacher and doula, Janine Rudin from Birth Basics.

“Parents need good information and support from their medical team as well as good antenatal education to help them feel more confident about birthing a bigger baby and knowing what is to be expected and considered normal for them.”