Arise from two separate fertilized ova (Zygotes) (Figure 26.1).
Each embryo with its own amnion, chorion, and yolk sac.
Dichorionic/diamniotic (DC/DA) twin pregnancy.
Two placentas or one fused placenta.
Monozygotic: Arise from the division of a single zygote.
Division of zygote during first 3 days of postconception.
Two embryos with two amnions and two chorions.
Two placenta or one fused placenta.
Division occurs b/w 4–8 days postconception.
Two embryos, two amnions, and two yolk sacs within a single (one) chorion.
Division after day 8 of postconception.
Two embryos within a single (one) amnion and single (one) chorion.