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Twin pregnancy refers to a gestation in which two fetuses develop simultaneously. It may arise from two separate ova (dizygotic) or one fertilized ovum that divides (monozygotic).
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https://www.youtube.com/watch?v=wUmUXknh_ls&list=PLy35JKgvOASnHHXni4mjXX9kwVA_YMDpq&index=13
https://www.youtube.com/watch?v=qU9EIVcOyB4
Twin pregnancies are associated with increased maternal, fetal, and perinatal risks, especially in monochorionic twins.
Classification by zygosity (Genetic Origin)
| Type | Description | Incidence |
|---|---|---|
| Dizygotic (fraternal) | Two ova fertilized by two sperm; genetically distinct | ~70% of twins |
| Monozygotic (identical) | Single ovum splits post-fertilization | ~30% of twins |
Classification by Chorionicity and Amnionicity (Membrane Sharing)
| Chorionicity–Amnionicity | Time of Zygote Split | Description |
|---|---|---|
| Dichorionic diamniotic (DCDA) | 0–3 days | 2 placentas, 2 amniotic sacs (can be monozygotic or dizygotic) |
| Monochorionic diamniotic (MCDA) | 4–8 days | 1 placenta, 2 amniotic sacs; risk of TTTS |
| Monochorionic monoamniotic (MCMA) | 8–13 days | 1 placenta, 1 amniotic sac; risk of cord entanglement |
| Conjoined twins | >13 days | Incomplete separation; shared organs |
| Feature | DCDA | MCDA | MCMA |
|---|---|---|---|
| Lambda (twin peak) sign | Present | Absent | Absent |
| Thin membrane | Thick | Thin | Absent |
| Number of placentas | 2 (or fused) | 1 | 1 |
| Gender | May be same or different | Always same | Always same |
I. Maternal Risks in Twin Pregnancy