Blastocyst culture is associated with an elevated incidence of monozygotic twinning after single embryo transfer.

In a 7-year (2002-2008) retrospective study of a large IVF program based on minimal ovarian stimulation and single ET (47,841 single ETs), monozygotic twinning occurred in 1.01% of 14,956 clinical pregnancies. Blastocyst culture was associated with a significantly increased monozygotic twinning risk (adjusted odds ratio, 2.04; 95% confidence interval, 1.29-4.48), whereas embryo freezing, type of stimulation protocol used, intracytoplasmic sperm injection fertilization, or zona removal did not influence its incidence.