Monozygotic (identical) twins and dizygotic (fraternal) twins.
- Monozygotic twins develop from a single zygote whose embryonic cells separate into two groups within about 8 days. They are genetically identical and always of the same gender. If the separation happens after 8 days, conjoined (Siamese) twins may form.
- Dizygotic twins develop when two oocytes are released and fertilized by two separate sperms. They are genetically different and may be of the same or different gender.