Monozygotic vs. dizygotic twins
| Identical twins (monozygotic twins) | Fraternal twins (dizygotic twins) | |
|---|---|---|
| Frequency | ⅓ of all twin pregnancies | ⅔ of all twin pregnancies |
| Origin | Division of the fertilized oocyte into two embryonic layers | Fertilization of two oocytes with two mature spermatozoa |
| Genetics of the individual | Genetically identical | Genetically different |
| Chorionic cavity and amniotic sac | Varies (see below) | Dichorionic-diamniotic |
Special features in the development of monozygotic twins
| Type | Description | Time of division of the zygote | Frequency in monozygotic twins |
|---|---|---|---|
| Dichorionic-diamniotic | The twins have separate chorionic sacs (separate placentas) and separate amniotic sacs. | Within the first 3 days after conception | ~20–30% |
| Monochorionic-diamniotic | The twins share a single chorionic sac (the twins share a placenta) but have a separate, individual amniotic sac. | Day 4–7 after conception | ~70% |
| Monochorionic-monoamniotic | The twins share a single chorionic sac (the twins share a placenta) and a single amniotic sac. | Day 8–11 after conception | ~1–5% |
| Monochorionic-monoamniotic (conjoined twins) | The twins share the placenta and amniotic sac, and are conjoined. | From day 12 after conception onwards | <0.1% |
take separate cars or share a CAB
- Splitting 0–4 days: separate chorion and amnion (di-di)
- Splitting 4–8 days: shared Chorion (mo-di)
- Splitting 8–12 days: shared chorion and Amnion (mo-mo)
- Splitting 13+ days: shared chorion, amnion, and Body (mo-mo; conjoined)