Fetal medicine · Monochorionic twin pregnancy

Twin-to-twin transfusion syndrome — surveillance for monochorionic-diamniotic twins

Twin-to-twin transfusion syndrome (TTTS) is a serious complication of monochorionic-diamniotic (MCDA) twin pregnancy in which blood flow becomes unbalanced between the twins through shared placental vessels. Without surveillance, MCDA twins who develop TTTS can deteriorate quickly. With structured fortnightly scanning by an experienced operator, TTTS is identified early and managed in time. This page describes how the centre surveys MCDA pregnancies and what happens when TTTS is detected.

Why MCDA twins need surveillance

Monochorionic-diamniotic twins share a single placenta with vascular anastomoses between the twins’ circulations. When inter-twin blood flow becomes imbalanced — one twin transfusing the other — the result can be twin-to-twin transfusion syndrome (TTTS), selective fetal growth restriction (sFGR), or twin anaemia-polycythaemia sequence (TAPS). All three are exclusively MCDA-pregnancy complications.

Up to 15 per cent of MCDA pregnancies develop TTTS. Without intervention, severe TTTS carries a very high perinatal mortality. With timely intervention, outcomes are substantially better. The surveillance pathway is therefore not optional.

Chorionicity determination at 11–14 weeks

Determining chorionicity is the single most important step in early twin pregnancy management. The window is 11 to 14 weeks — later in pregnancy, chorionicity is much harder to determine reliably. The features assessed are the lambda sign (dichorionic) or T sign (monochorionic), the number of placental masses, and the inter-twin membrane thickness. Documentation should be unambiguous.

Surveillance schedule for MCDA pregnancies

• From 16 weeks: fortnightly ultrasound — the standard surveillance interval
• Each scan documents both twins’ biometry, deepest vertical pocket of amniotic fluid in each sac, bladder visibility in each twin, and middle cerebral artery (MCA) peak systolic velocity from approximately 20 weeks
• Discordance in fetal growth, amniotic fluid, or MCA-PSV is the trigger for closer evaluation
• Fetal cardiac function is assessed when TTTS is suspected
• Detailed anatomical survey at 18–22 weeks as for all pregnancies

Quintero staging of TTTS

• Stage I — oligohydramnios in donor (DVP <2 cm) + polyhydramnios in recipient (DVP >8 cm). Bladders visible in both. Normal Dopplers.
• Stage II — as Stage I + donor bladder not visible
• Stage III — as Stage II + critically abnormal Dopplers (absent or reversed end-diastolic flow in umbilical artery, abnormal ductus venosus, pulsatile umbilical vein) in either twin
• Stage IV — hydrops in either twin
• Stage V — demise of one or both twins

Staging guides management and is repeated at each scan.

Management

• Stage I — often managed with intensified surveillance; selected cases progress and may need intervention
• Stage II onwards — fetoscopic laser ablation of placental anastomoses is the gold-standard treatment in expert centres, ideally between 16 and 26 weeks
• Late-onset TTTS — managed by amnioreduction or selective options per gestation
• Stage IV or V — individualised management; cord occlusion may be discussed in selected cases
• Steroids for fetal lung maturation when delivery is approaching

Related complications to screen for

• Selective fetal growth restriction (sFGR) — growth discordance >25 per cent with one twin’s estimated weight below the 10th centile
• Twin anaemia-polycythaemia sequence (TAPS) — MCA-PSV discordance without amniotic-fluid discordance; can be spontaneous or post-laser
• Single intra-uterine demise — carries risk to the surviving co-twin

Counselling and communication

The chorionicity result and surveillance plan are explained to the couple at the 11–14 week visit. The conversation includes the natural history, the rationale for fortnightly scans, what TTTS looks like, what would trigger escalation, and the referral pathway for fetoscopic laser where indicated.

When to refer

• Any MCDA pregnancy from 16 weeks if surveillance is not in place locally
• Suspected TTTS at any stage
• Suspected sFGR with abnormal Dopplers
• Single fetal demise in a monochorionic pregnancy
• Couples asking for an opinion before deciding on management

Guidelines we follow on this topic

• ISUOG Practice Guideline on Twin Pregnancy
• RCOG Green-top on Multiple Pregnancy
• SMFM Consult Series on TTTS
• FIGO good practice on twin pregnancy

Related reading

• Fetal medicine programme
• Fetal medicine scans
• Fetal medicine conditions
• Pregnancy care
• High-risk pregnancy
• Menstrual cycle physiology

TTTS surveillance in monochorionic twin pregnancies requires fortnightly ultrasound from 16 weeks. Early detection via Quintero staging guides timing of intervention – fetoscopic laser is the treatment of choice for stage II-IV TTTS before 26 weeks.

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Explore the Fetal Medicine Programme

Twin-to-Twin Transfusion Syndrome is one element of the broader fetal medicine programme. The main fetal medicine pillar covers the complete pregnancy ultrasound schedule, anomaly screening, and Doppler protocols.

Dr. Priyadatt Patel

Senior Gynecologist · Advanced Laparoscopic Surgeon · IVF and Endometriosis Programme Lead

MS OBGyn · Pregnancy Care · Advanced Gynaecological Ultrasound · Fertility Preservation

ESHRE / ESGE / AAGL / ASRM guideline-aligned practice. 3D Karl Storz precision technique. Fertility-preservation-first philosophy. Evidence-based decisions, honest counselling, long-term outcomes orientation.