Twin Pregnancy Risk Assessment

MCDA (monochorionic diamniotic) twins share a placenta but have separate amniotic sacs. They account for approximately 70% of identical (monozygotic) twin pregnancies. MCMA (monochorionic monoamniotic) twins share both the placenta and a single amniotic sac and are much rarer — they carry additional risks of cord entanglement. TTTS occurs in MCDA twins; MCMA twins face different complications.

Both conditions occur in MCDA twins but have different mechanisms. TTTS is caused by unbalanced arteriovenous anastomoses producing amniotic fluid discordance. Selective fetal growth restriction (sFGR) occurs when one twin has a significantly smaller portion of the shared placenta, leading to growth restriction without the characteristic fluid discordance of TTTS. The two conditions can coexist.

The Solomon technique for fetoscopic laser surgery involves coagulation of all visible anastomoses on the placental surface, followed by coagulation of the tissue between all the anastomoses to create a complete vascular equatorial demarcation. This 'dichorionisation' of the placenta has been shown to reduce the risk of post-laser TAPS compared to selective laser coagulation of individual anastomoses.

A cervical length below 25 mm on transvaginal ultrasound at or before 24 weeks is the standard threshold for concern in MCDA twins with TTTS. Short cervix predicts preterm birth and may lead to cervical cerclage or progesterone therapy in addition to TTTS treatment. It also influences timing and urgency of laser surgery.

Major studies including the Eurofetus trial and TAPS follow-up data show that approximately 85% of TTTS twins surviving after laser have no major neurodevelopmental impairment. The recipient twin has slightly higher risk of cardiac dysfunction and polycythaemia-related complications. The donor twin has higher risk of anaemia and growth restriction. Overall, long-term outcomes are significantly better with laser than with amnioreduction.

Laser surgery is most effective between 16 and 26 weeks gestation. Before 16 weeks, the fetoscope cannot safely enter the amniotic cavity. After 26 weeks, amnio-reduction may provide temporary relief, and early delivery is considered if the fetus is at risk. Some centres with experience may offer laser up to 28 weeks in selected cases.

True TTTS recurrence after technically successful laser is rare (<5%) but possible if residual anastomoses remain. However, new complications such as TAPS (from microanastomoses) or reverse TTTS (where previously donor becomes recipient) can occur in 10–15% of cases post-laser. Close post-laser surveillance with MCA Doppler studies is essential. This is particularly important in the context of twin to twin risk calculations, where accuracy directly impacts decision-making. Professionals across multiple industries rely on precise twin to twin risk computations to validate assumptions, optimize processes, and ensure compliance with applicable standards. Understanding the underlying methodology helps users interpret results correctly and identify when additional analysis may be warranted.

In MCDA pregnancy, the death of one twin carries significant risk to the co-twin — approximately 20–40% risk of intrauterine death of the survivor and 25–35% risk of serious neurological injury due to acute haemodynamic shifts through shared placental anastomoses. Urgent delivery and/or cord occlusion of the deceased twin may be considered depending on gestation. Neurodevelopmental follow-up of the survivor is essential.