What is the management of monochorionic (mono-chorionic) twin pregnancies?

Management of Monochorionic Twin Pregnancies

All monochorionic twin pregnancies require ultrasound surveillance every 2 weeks starting at 16 weeks of gestation until delivery to screen for twin-twin transfusion syndrome (TTTS) and other complications. 1

Initial Assessment and Chorionicity Determination

Establish chorionicity and amnionicity by first-trimester ultrasound (10-13 weeks of gestation) to identify monochorionic pregnancies, which carry 2-fold higher perinatal mortality compared to dichorionic twins. 1 This early determination is critical because monochorionic twins share a single placenta with vascular anastomoses present in >95% of cases, predisposing them to unique complications. 1

First Trimester Evaluation Should Include:

• Confirmation of monochorionic-diamniotic (MCDA) placentation 1
• Nuchal translucency (NT) screening 1
• Crown-rump length measurement 1
• Documentation of NT abnormalities or crown-rump length discrepancy, which increase TTTS risk and may warrant weekly rather than biweekly surveillance 1

Surveillance Protocol Starting at 16 Weeks

Minimum Required Assessments Every 2 Weeks:

• Maximum vertical pocket (MVP) measurement in each amniotic sac 1
• Visualization of urine-filled bladder in each fetus 1
• Umbilical artery Doppler studies (ideally incorporated into routine surveillance) 1
• Middle cerebral artery (MCA) peak systolic velocity (PSV) Doppler starting at 16 weeks to screen for twin anemia-polycythemia sequence (TAPS) 1

Critical pitfall: Surveillance intervals longer than 2 weeks are associated with higher rates of late-stage TTTS diagnosis, which significantly worsens outcomes. 1

Diagnostic Criteria for Major Complications

Twin-Twin Transfusion Syndrome (TTTS):

TTTS is diagnosed when MVP <2 cm in one sac (donor twin with oligohydramnios) AND MVP >8 cm in the other sac (recipient twin with polyhydramnios). 1

Twin Anemia-Polycythemia Sequence (TAPS):

TAPS requires MCA-PSV >1.5 multiples of the median (MoM) in the donor twin AND <1.0 MoM in the recipient twin, OR an intertwin difference >0.5 MoM. 1

Management Algorithm for TTTS

Stage I TTTS (16-26 weeks):

• Expectant management with at least weekly fetal surveillance for asymptomatic patients 1
• Consider fetoscopic laser surgery if maternal symptoms from polyhydramnios develop 1

Stage II-IV TTTS (16-26 weeks):

Fetoscopic laser surgery is the standard treatment and all patients should be referred to a fetal intervention center. 1 This represents the highest quality evidence (GRADE 1A) in monochorionic twin management. 1

Post-Laser Surveillance:

• Weekly ultrasounds for 6 weeks after laser therapy 1
• Resume every-other-week surveillance thereafter unless concerns arise for post-laser TTTS, post-laser TAPS, or fetal growth restriction 1

Management Algorithm for TAPS

Before 32 Weeks:

• Stage I TAPS: Close monitoring with serial ultrasounds rather than intervention 2
• Stage II or higher TAPS: Mandatory referral to specialized fetal care center for laser therapy evaluation 1, 2

At or After 32-34 Weeks:

Delivery is preferred over laser therapy for stage I TAPS at this gestational age. 2

Important caveat: TAPS management is based on lower-quality evidence than TTTS and requires individualized decision-making at specialized centers, particularly for early-onset severe disease. 2

Delivery Timing

Uncomplicated MCDA Pregnancies:

Deliver at 34-36 weeks of gestation. 1

After Successful Laser Therapy with Both Twins Surviving:

Deliver at 34-36 weeks of gestation. 1

After Single Fetal Demise Post-Treatment:

Deliver the surviving co-twin at 39 weeks (full-term) to avoid complications of prematurity unless other indications for earlier delivery exist. 1 This recommendation differs from the earlier delivery timing in uncomplicated MCDA twins because the risk of acute twin-to-twin transfusion through anastomoses has been eliminated by laser therapy.

Mode of Delivery:

Fetoscopic laser surgery should not influence the mode of delivery. 1

Additional Monitoring Considerations

High-Risk Features Warranting More Frequent Surveillance:

• Velamentous cord insertion (found in approximately one-third of TTTS placentas) 1
• Intertwin membrane folding (associated with TTTS development in >33% of cases) 1
• Umbilical artery Doppler abnormalities, especially with fluid or growth discordance 1

Key principle: The dynamic nature of TTTS—which can remain stable, regress spontaneously, or progress rapidly—necessitates consistent biweekly surveillance throughout the second and third trimesters until delivery. 1 Approximately 30% of monochorionic pregnancies develop complications requiring specialized management, with 15% eligible for invasive fetal therapy. 3