Monozygotic twinning and IVF/ICSI treatment: a report of 11 cases and review of literature.

Monozygotic twinning is a relatively rare event in in-vivo conception, being estimated to occur in 0.42% of all births. The underlying mechanism for monozygotic twin formation is the division of the embryo early in its development. Separation of cells may theoretically occur before or after inner cell mass formation. In this analysis we report 11 cases of monozygotic twins resulting from IVF or intracytoplasmic sperm injection (ICSI) treatment cycles performed between 1991 and 1998 at St James's University Hospital, Leeds, and Bourn Hall Clinic, Cambridge, UK. Six cases (55%) followed treatment with ICSI and seven cases (64%) were in women aged > or = 35 years. This article also reviews the scientific literature discussing information pertaining to frequency, aetiology and potential complications of the monozygotic twinning phenomenon. We conclude that patients at risk of monozygotic twinning are those aged >35 years of age and those who had artificial opening in the zona pellucida by application of micromanipulation techniques. Women undergoing assisted conception treatment, particularly those with these two risk factors, must be informed of the low but definite risk of monozygotic twinning when counselled regarding the number of embryos to be transferred and chances of multiple births.