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Twins occur in about 1% of all pregnancies in which 30% are identical (maternal, monozygotic) twins and 70% are non-identical (fraternal, dizygotic) twins.

A single baby is formed when an egg cell is fertilized by a single sperm cell to form a zygote. The zygote divides to form a structure composed of hundreds of cells called a blastocyst. The blastocyst implants into the uterine lining and will grow into a single baby.

Identical twins start out from a single fertilized egg cell (zygote). Unlike a single baby, the fertilized egg cell will split into two separate embryos during the two-cell stage (day 2), early blastocyst stage (day 4), or late blastocyst stage (day 6).

The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. The earlier the splitting occurs, the more independently the twins will develop in the uterus. Twins that split during the late blastocyst stage will share an amnion, chorion, and amniotic sac.

Non-identical twins develop from two fertilized egg cells (zygotes). During ovulation, two egg cells are released and fertilized by two different sperm cells. Non-identical twin embryos develop separately each having their own chorion, amnion, and placenta.

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