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Foundation Sciences · Embryology
Twinning
Twin pregnancies arise from dizygotic (fraternal) or monozygotic (identical) origins, with chorionicity determined by timing of splitting in monozygotic twins.
📌 Learning Objectives
- Describe the underlying mechanism of Twinning.
- Identify the key clinical features and complications of Twinning.
- Outline the appropriate investigations and management of Twinning.
- Discuss the implications for patients and families of Twinning.
📋 Overview
Dichorionic diamniotic (DCDA) twins have separate placentas; monochorionic diamniotic (MCDA) share a placenta with two amniotic sacs. Monochorionic monoamniotic and conjoined twins arise from late splitting (>8 days).
🔬 Basic Science
Dichorionic diamniotic (DCDA) twins have separate placentas; monochorionic diamniotic (MCDA) share a placenta with two amniotic sacs. Monochorionic monoamniotic and conjoined twins arise from late splitting (>8 days).
🏥 Clinical Relevance
Chorionicity is best determined on first trimester ultrasound by the lambda or T sign.
🧪 Investigations
Investigation depends on clinical context: relevant blood tests, imaging, and specific genetic or histopathological tests as appropriate. Refer to specialist services where indicated.
💊 Management
Management is condition-specific and typically multidisciplinary, combining medical therapy, surgical intervention where appropriate, supportive care, and family/genetic counselling.
Revision Resources – expand the sections below for high-yield notes, exam pearls, key facts and further reading.
MLA High-Yield Notes & Quick Revision ⌄
Common SBA themes: recognising the underlying mechanism, identifying classic clinical features, and choosing the first-line investigation or management step. Watch for inheritance pattern and characteristic associations.
twinning
monozygotic
dizygotic
ttts
chorionicity
- Dizygotic twins are always dichorionic diamniotic.
- Monozygotic twins may be DCDA, MCDA or MCMA depending on the day of splitting.
- Splitting day 0–3: DCDA; day 4–8: MCDA; day 8–13: MCMA; >13: conjoined.
- TTTS is a complication of monochorionic pregnancies due to placental anastomoses.
- The lambda sign on ultrasound indicates dichorionicity.
Exam Pearls ⌄
⭐ High Yield
Dizygotic twins are always dichorionic diamniotic.
Monozygotic twins may be DCDA, MCDA or MCMA depending on the day of splitting.
Splitting day 0–3: DCDA; day 4–8: MCDA; day 8–13: MCMA; >13: conjoined.
TTTS is a complication of monochorionic pregnancies due to placental anastomoses.
The lambda sign on ultrasound indicates dichorionicity.
💡 Clinical Pearl
Twinning: Chorionicity is best determined on first trimester ultrasound by the lambda or T sign.
⚠️ Exam Tip — Common Mistakes
Confusing the mechanism of Twinning with related conditions.
Missing classic clinical features of Twinning in SBA stems.
Failing to consider Twinning in the differential diagnosis.
Key Facts ⌄
Dizygotic twins are always dichorionic diamniotic.
Monozygotic twins may be DCDA, MCDA or MCMA depending on the day of splitting.
Splitting day 0–3: DCDA; day 4–8: MCDA; day 8–13: MCMA; >13: conjoined.
TTTS is a complication of monochorionic pregnancies due to placental anastomoses.
The lambda sign on ultrasound indicates dichorionicity.
Related Topics ⌄
References ⌄
- GMC MLA Content Map
- NICE Clinical Knowledge Summaries
- BMJ Best Practice
Further Resources
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