In monozygotic twin pregnancies, what is the most common type of placentation?

Explanation

## Placentation in Monozygotic Twins **Key Point:** The type of placentation in monozygotic (MZ) twins depends on the timing of embryonic division after fertilization. ### Timing of Division and Placentation | Timing of Division | Chorionicity | Amnionicity | Frequency in MZ Twins | |---|---|---|---| | Day 0–3 (blastocyst stage) | Dichorionic | Diamniotic | ~25% | | Day 4–8 (early embryonic disc) | **Monochorionic** | **Diamniotic** | **~70%** | | Day 8–13 (late embryonic disc) | Monochorionic | Monoamniotic | ~2% | | Day 13+ (primitive streak) | Conjoined twins | — | <1% | **High-Yield:** Monochorionic diamniotic is the **most common placentation in MZ twins** (~70%), occurring when division happens between days 4–8 after fertilization. ### Clinical Significance **Clinical Pearl:** Monochorionic placentation carries increased risks: - Twin-to-twin transfusion syndrome (TTTS) — unique to monochorionic twins - Selective intrauterine growth restriction (sIUGR) - Cord entanglement (if monoamniotic) - Higher perinatal morbidity and mortality **Mnemonic:** **MOMO** = **MO**nochorionic **MO**noamniotic (rarest, highest risk; ~2%) ### Ultrasound Diagnosis - **Dichorionic:** Two separate placental discs or "twin peak sign" (echogenic projection at placental junction) - **Monochorionic diamniotic:** Single placenta with **intertwin membrane** (thin, two-layer membrane; no twin peak sign) - **Monochorionic monoamniotic:** Single placenta, **no membrane** between twins [cite:Williams Obstetrics 26e Ch 45]