In vitro fertilisation (IVF) babies grown from frozen embryos are more likely to be oversized, research has shown.
Frozen embryos increase the risk of a baby being born too heavy 1.6-fold compared with “fresh” embryos.
Compared with naturally conceived children, the risk is 1.5 times higher.
The same study found the use of frozen embryos greatly reduced the risk of having a baby that is too small.
High birth weight babies are more likely to be delivered by Caesarean section or need other kinds of intervention that may lead to complications.
Frozen embryo transfer (FET) involves the implantation into the womb of embryos that have been frozen, stored and thawed.
The Danish study compared 910 FET babies with 9,603 babies born after “fresh” embryo transfer and 4,656 naturally conceived (NC) babies.
The proportion of large-for-gestational age (LGA) babies was 16.9% for the FET group, compared with 10.3% for the fresh transfer babies and 11.4% for NC children.
Dr Anja Pinborg, from the University of Copenhagen, who led the research, said: “Cryopreservation (freezing) of embryos can result in ‘large offspring syndrome’ which may be explained by epigenetic changes (genetic alterations caused by the environment) in the very early embryonic stages caused by freezing and thawing procedures.”
The findings were presented at the annual meeting of the European Society of Human Reproduction and Embryology (Eshre) in Stockholm, Sweden.