Overview of anatomy. Uterus, fallopian tube and ovary. Shows steps of ovulation, fertilization and early development. Most twinning occurs between first 30 hours and 5-6 days. From Langman’s Medical Embryology.

How identical twins can be formed. A: divides at two cell stage (most identical). B: divides at about 5 days, creates mirror image identical twins (see placenta below). C: divides around day 9, mirror image twins, but now only one sac (dangerous because cords can become tangled). If division happens after day 10, create conjoined twins (see xray below). From Langman’s Medical Embryology.

Monochorionic diamnionic twin placenta, like column B above. ©Harvey Kliman, MD, PhD

Conjoined twins, 18 weeks, xray. ©Harvey Kliman, MD, PhD

Detail of fertilization and then two cell stage where the cells are already starting to be different (shown as red versus green cells). The red cell has different genes turned on compared to the green cell. ©Harvey Kliman, MD, PhD

Diagram showing how the two cell embryo with one red and one green cell makes a ball of cells, half red, half green. If this grape-like cluster splits down the middle into one red half and one green half, each half will make a baby with different gene expression, even though they are "identical twins" (red diaper versus green diaper). ©Harvey Kliman, MD, PhD

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Last modified 3.10.05