Monozygous double inner cell masses in mouse blastocysts following fertilization in vitro and in vivo.

The incidence of human identical twins after in vitro fertilization and embryo transfer (IVF-ET) is estimated to be higher than the commonly accepted incidence after conception in vivo. This paper reports that 8 mouse blastocysts which had double inner cell masses in 261 blastocysts (3.1%) were identified after fertilization in vitro and they developed into trophoblastic outgrowth formation with two inner cell masses. In contrast, only 3 monozygous double inner cell masses were observed in 526 blastocysts (0.6%) which were fertilized in vivo and cultured from the two-cell stage in vitro. It is therefore possible that fertilization in vitro predisposes to monozygous twinning.