Management of Monochorionic Twin Pregnancy
Background
Monochorionic (MC) placentation occurs in ~30% of twin pregnancies and carries near-universal vascular anastomoses connecting the two fetal circulations. This shared circulation is the root cause of complications unique to monochorionicity: twin-to-twin transfusion syndrome (TTTS, ~15% of MC twins), selective growth restriction (sGR, ~10–15%), twin anaemia-polycythaemia sequence (TAPS, ~2% of uncomplicated MC diamniotic twins, up to 13% post-laser ablation), and twin reversed arterial perfusion (TRAP, ~1%). MC twins have higher fetal loss rates than dichorionic twins, mainly from second-trimester loss, and higher neurodevelopmental morbidity risk.
Diagnosis and Early Pregnancy
Chorionicity must be determined by ultrasound between 11+0 and 13+6 weeks (crown–rump length 45–84 mm) — accuracy is highest before 14 weeks. MC pregnancies show a single placental mass and a thin inter-twin membrane inserting perpendicularly (T-sign), versus the lambda sign in dichorionic twins. A photographic record of the membrane insertion should be retained. If chorionicity is uncertain, manage as monochorionic until proven otherwise.
Surveillance Schedule
- Detailed anomaly scan at 18+0–20+6 weeks, including extended fetal cardiac views.
- Ultrasound every 2 weeks from 16+0 weeks until delivery in uncomplicated MC pregnancies (TTTS can rarely present after 26 weeks, so surveillance continues to term).
- At every scan: liquor volume in each sac (deepest vertical pocket, DVP), umbilical artery pulsatility index, and fetal bladder visualisation.
- From 16+0 weeks, estimated fetal weight (EFW) and inter-twin EFW discordance are calculated at each visit to screen for sGR.
- TAPS is screened for (serial MCA peak systolic velocity, MCA-PSV) after laser ablation for TTTS and in other complicated MC pregnancies referred to a fetal medicine centre.
Twin-to-Twin Transfusion Syndrome (TTTS)
Defined by significant amniotic fluid discordance and staged using the Quintero system:
- Stage I: Oligohydramnios (DVP <2 cm) in the donor sac with polyhydramnios (DVP >8 cm before 20 weeks, >10 cm after 20 weeks) in the recipient sac; donor bladder visible, normal Dopplers.
- Stage II: Donor bladder not visible (anuria); Dopplers not critically abnormal.
- Stage III: Critically abnormal Dopplers (donor umbilical artery and/or recipient ductus venosus/umbilical vein).
- Stage IV: Hydrops (ascites, effusions, scalp oedema) usually in the recipient.
- Stage V: Death of one or both twins.
TTTS presenting before 26 weeks should be treated with fetoscopic laser ablation (ideally the Solomon technique) rather than amnioreduction or septostomy, performed at centres doing ≥15 procedures/year. Post-treatment, weekly ultrasound (fetal brain, heart, limbs, Dopplers) is recommended, extending to 2-weekly after 2 weeks if growth is adequate. Delivery of treated TTTS pregnancies is recommended between 34+0 and 36+6 weeks.
Selective Growth Restriction (sGR)
Defined as EFW discordance >20%. Classified by umbilical artery Doppler:
- Type I: Positive end-diastolic velocities in both twins.
- Type II: Absent/reversed end-diastolic velocities (AREDV) in the smaller twin.
- Type III: Cyclical/intermittent AREDV — carries risk of unpredictable acute deterioration.
Surveillance is at least 2-weekly with Doppler. Type I: delivery by 34–36 weeks if growth velocity and Dopplers are reassuring. Type II/III: delivery by 32 weeks unless deterioration mandates earlier action.
Twin Anaemia-Polycythaemia Sequence (TAPS)
Diagnosed by MCA-PSV: donor >1.5 multiples of the median (MoM, suggesting anaemia), recipient <1.0 MoM (suggesting polycythaemia), without the significant oligo-/polyhydramnios seen in TTTS. Natural history and optimal management remain poorly established.
Delivery Timing (Uncomplicated MC Twins)
Elective delivery from 36+0 weeks with antenatal corticosteroids, absent earlier indications. Vaginal birth is appropriate for MC diamniotic twins without other obstetric contraindications. Monochorionic monoamniotic (MCMA) twins should be delivered by caesarean section between 32+0 and 34+0 weeks given the high risk of cord entanglement and fetal death.
High-Yield Exam Points
- Chorionicity is best determined by 11–13+6 weeks via T-sign (MC) vs lambda sign (dichorionic); manage as MC if uncertain.
- TTTS staged with Quintero I–V; DVP <2 cm (donor) / >8 cm pre-20wks or >10 cm post-20wks (recipient) defines Stage I.
- First-line TTTS treatment <26 weeks = fetoscopic laser ablation (Solomon technique), not amnioreduction/septostomy.
- TAPS = donor MCA-PSV >1.5 MoM + recipient MCA-PSV <1.0 MoM, without TTTS-pattern liquor discordance.
- sGR = EFW discordance >20%; Type II/III (AREDV/cyclical) carries highest risk and prompts delivery by 32 weeks.
- Uncomplicated MC diamniotic twins: deliver from 36+0 weeks; MCMA twins: caesarean at 32–34 weeks.
- Single fetal death in MC twins carries ~15% co-twin death risk and ~26% neurological abnormality risk in survivors.
- Selective feticide by intravascular injection is not an option in MC twins due to shared placental circulation; cord occlusion techniques are used instead.
Source: RCOG Green-top Guideline No. 51 (Third edition — 2024 partial update (Kilby MD, Bricker L, BJOG 2025;132). Supersedes the second edition (November 2016) and first edition (December 2008). The 2024 update addressed cell-free fetal DNA screening, and changes to surveillance/management of selective growth restriction (sGR) and twin anaemia-polycythaemia sequence (TAPS); core diagnostic and staging content below is drawn from the verified second-edition text (BJOG 2016;124:e1–e45), which remains the basis of current practice.)
Read the original on rcog.org.uk
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