What is the recommended management and surveillance for a monochorionic diamniotic twin pregnancy?

Management and Surveillance of Monochorionic Diamniotic Twin Pregnancies

Begin ultrasound surveillance at 16 weeks' gestation and continue every 2 weeks until delivery to screen for twin-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and selective intrauterine growth restriction (sIUGR). 1, 2

Initial Assessment and Chorionicity Determination

Establish chorionicity ideally between 10-13 weeks' gestation using first-trimester ultrasound, as this is the single most important prognostic factor in twin pregnancies. 1, 2, 3 Monochorionic twins account for 20-30% of all twin pregnancies but represent 30% of all complications, with an overall mortality rate of approximately 10%. 1

First Trimester Evaluation (11-14 weeks)

• Perform nuchal translucency (NT) measurement and crown-rump length (CRL) assessment for both twins 1
• NT discordance ≥20% carries up to 30% risk of severe TTTS or early intrauterine fetal demise and warrants specialist referral 1
• CRL discordance ≥10% is associated with increased pregnancy loss, though predictive value is only 52% 1

Surveillance Protocol by Gestational Age

Starting at 16 Weeks Through Delivery

Perform ultrasound every 2-3 weeks (many experts recommend every 2 weeks) with the following assessments: 1, 2

• Fetal biometry to calculate estimated fetal weight (EFW) and assess for growth discordance 1, 4
• Amniotic fluid volume in each sac using maximal vertical pocket (MVP) measurements 1
• Umbilical artery Doppler studies to evaluate placental function 1, 4
• Middle cerebral artery (MCA) peak systolic velocity to screen for TAPS 1

Second Trimester Anatomic Survey (18-22 weeks)

• Complete fetal anatomic assessment with particular attention to cardiovascular anomalies, which occur twice as frequently in monochorionic twins 1, 5
• Fetal echocardiography is specifically recommended for all monochorionic twins due to elevated cardiac defect risk 1
• Evaluate for velamentous cord insertion and vasa previa, both more common in monochorionic pregnancies and associated with adverse outcomes 1, 4
• Transvaginal cervical length assessment to stratify preterm delivery risk 1, 4

Specific Complications and Management Thresholds

Twin-Twin Transfusion Syndrome (TTTS)

TTTS occurs in 8-10% of monochorionic diamniotic pregnancies and requires two diagnostic criteria: 1

1. Confirmed monochorionic diamniotic placentation
2. Oligohydramnios (MVP ≤2 cm) in donor sac AND polyhydramnios (MVP ≥8 cm) in recipient sac 1

Management by Quintero stage: 1

• Stage I TTTS: More than 75% remain stable or regress without intervention; perinatal survival approximately 86%; expectant management is often appropriate 1
• Stage II-IV TTTS: Immediately refer to fetal intervention center for evaluation and consideration of fetoscopic laser photocoagulation of placental anastomoses 1, 6
• Advanced stage TTTS (especially stage III) presenting at ≤26 weeks has 70-100% perinatal loss without intervention 1

Twin Anemia-Polycythemia Sequence (TAPS)

• Occurs spontaneously in approximately 5% of monochorionic diamniotic twins 1
• Diagnosed by MCA-PSV >1.5 multiples of median in donor and <1.0 multiples of median in recipient, WITHOUT oligohydramnios-polyhydramnios 1
• If TAPS progresses to stage II before 32 weeks, refer to specialized fetal care center 6

Selective Intrauterine Growth Restriction (sIUGR)

Define sIUGR as EFW discordance >25% between twins OR one twin with EFW <10th percentile: 1, 4

Umbilical artery Doppler classification for risk stratification: 4

• Type 1 (constant end-diastolic flow): Best prognosis; weekly assessment of fetal well-being with umbilical artery and MCA Doppler, biweekly biometry 1
• Type 2 (constant absent/reversed end-diastolic flow): High risk for neonatal complications and potential twin demise 4
• Type 3 (intermittent absent/reversed end-diastolic flow): Variable outcomes; high risk of acute exsanguination if smaller twin dies 1, 4

When absent or reversed end-diastolic flow is present, there is high risk of intrauterine fetal demise of the growth-restricted twin and potential neurological morbidity in the surviving twin. 1

Single Fetal Demise

If one monochorionic twin dies spontaneously, the surviving cotwin may acutely exsanguinate into the demised twin through placental anastomoses, resulting in death or severe brain damage. 5 Immediate fetal MRI assessment is helpful for evaluating intracranial injury in the surviving twin. 1

Delivery Timing

Uncomplicated Monochorionic Diamniotic Twins

• Contemporary data show 98.7% likelihood of progressing from 24 weeks to 2 live births in uncomplicated pregnancies 7
• Risk of third-trimester fetal loss is low in uncomplicated cases 7
• These data do not support elective preterm delivery solely for prevention of intrauterine fetal demise in uncomplicated monochorionic diamniotic twins 7

Complicated Pregnancies

• After successful laser treatment for TTTS with both twins surviving, delivery timing may be extended to 34-36 weeks 6
• Administer corticosteroids for fetal lung maturation at 24 0/7 to 33 6/7 weeks, particularly in stage III TTTS and those undergoing invasive interventions 1

Critical Pitfalls to Avoid

• Do not rely on biophysical profile testing in twins with reactive non-stress tests and no other risk factors—insufficient evidence supports routine use 4
• Do not attribute growth discordance alone to TTTS—growth discordance and IUGR are not diagnostic criteria for TTTS, which specifically requires the oligohydramnios-polyhydramnios sequence 1
• Do not discontinue surveillance after 32 weeks—while fetal demise risk decreases, continued monitoring remains necessary 4
• Do not miss velamentous cord insertion, which increases risk of sIUGR, lower gestational age at birth, and intrauterine fetal demise 4