Dive Deeper

Identical and non-identical twins form differently. Non-identical twins form when the woman ovulates more than one egg, and each egg gets fertilized, forming two separate zygotes. Alternatively, one zygote can become two people for up to 10 days after fertilization. This happens in one of two ways. Early in development, one cell may split off from the rest of the cells in the embryo and start growing as a separate embryo.1

Identical twins come from one zygote that splits into two individual embryos. Fraternal twins come from two separate eggs that get fertilized separately. (Image Credit: Science Source)
When do twins form in the womb?

One-third of identical twins form during the first 3 days after fertilization. Another two-thirds of identical twins happen when the blastocyst (when the embryo is a hollow ball of cells) divides into two roughly equal halves, 4 to 8 days after fertilization.2

3D ultrasound image of 12 week old twins. Every time one twin moves, that twin may touch the other. Twins that touch each other provide some of the ultrasound evidence that the fetus moves when touched in the womb. (Image Credit: Science Source)