According to several studies, twin pregnancy is more likely to occur in in vitro fertilisation (IVF) treatments than naturally. The exact cause of this is unknown, although there are thought to be several hypotheses.
It is commonly believed that the zona pellucida of the embryo plays a key role. It is possible that when the embryo hatches to emerge and implant in the uterus it splits in two. This process could be triggered by several factors such as embryo manipulation for ICSI and assisted hatching to promote embryo release.
In addition, hardening of the pellucid layer and embryo culture in the laboratory could also be reasons for this increased likelihood of having twins in IVF.