Superfecundation refers to

Mechanism of monozygotic twinning:The outcome of the monozygotic twinning process depends on when division occurs. If zygotes divide within the first 72 hours after feilization, two embryos, two amnions, and two chorions develop, and a diamniotic, dichorionic twin pregnancy evolves.If division occurs between the fouh and eighth day, a diamniotic, monochorionic twin pregnancy results. By approximately 8 days after feilization, the chorion and the amnion have already differentiated, and division results in two embryos within a common amniotic sac, that is, a monoanionic, monochorionic twin pregnancy. Conjoined twins result if twinning is initiated later.(Ref: William's Obstetrics; 25th edition)