What is the significance of the OBS (Obstetric Score) in twin pregnancies?

OBS Score in Twin Pregnancies: Significance and Monitoring Protocols

In twin pregnancies, the OBS (Obstetric Score) is critical for risk stratification and surveillance, with chorionicity being the most important determinant of monitoring frequency, as monochorionic twins require biweekly ultrasound surveillance starting at 16 weeks gestation to detect complications like TTTS early.

Understanding Chorionicity and Its Impact

• Chorionicity (whether twins share a placenta) is the most important determinant of pregnancy risk and should be established in the first trimester when accuracy is highest 1
• Monochorionic twins (sharing one placenta) represent 20% of twin pregnancies but account for 30% of all complications and have an overall mortality rate of approximately 10% 1
• Dichorionic twins (separate placentas) have lower complication rates but still require more intensive monitoring than singleton pregnancies 1

Surveillance Protocols Based on Chorionicity

For Monochorionic Twins:

• Begin ultrasound surveillance at 16 weeks gestation 1
• Perform ultrasound evaluations every 2 weeks until delivery 1
• Each evaluation should include:
  • Maximum vertical pocket (MVP) of amniotic fluid in each sac 1
  • Presence of fluid-filled urinary bladder in each fetus 1
  • Umbilical artery Doppler assessment, especially with discordant fluid or growth 1

For Dichorionic Twins:

• Less intensive monitoring is required compared to monochorionic twins 1
• Typically followed every 3-4 weeks in uncomplicated cases 1
• Closer surveillance if growth discordance or other complications develop 1

Key Parameters to Monitor

• Growth Assessment:

  • Monitor for discordant growth (defined as mild if 15% difference, moderate if 20%, severe if ≥25%) 1
  • Consider using twin-specific growth charts rather than singleton standards 2

• Amniotic Fluid Assessment:

  • For monochorionic twins: MVP >2cm and <8cm in each sac is normal 1
  • Diagnosis of TTTS: MVP <2cm in one sac and MVP >8cm in other sac 1

• Doppler Studies:

  • Umbilical artery Doppler is essential for monochorionic twins 1
  • Middle cerebral artery Doppler helps detect TAPS (Twin Anemia-Polycythemia Sequence) 1

Complications Requiring Enhanced Surveillance

• Twin-Twin Transfusion Syndrome (TTTS):

  • Occurs in monochorionic twins due to unequal blood flow through placental anastomoses 1
  • Weekly surveillance may be more effective than biweekly for early detection 1

• Selective Intrauterine Growth Restriction (sIUGR):

  • Common definition: one fetus has estimated fetal weight <10th percentile 1
  • Weekly assessment of fetal well-being with Doppler studies is recommended 1

• Discordant Growth:

  • Abdominal circumference discordance >10% between 14-22 weeks is predictive of adverse outcomes 1
  • Requires more frequent monitoring and Doppler assessment 1

Additional Monitoring Considerations

• Cervical Length Assessment:

  • Transvaginal ultrasound for cervical length helps identify risk of preterm delivery 1
  • Cervical length <1.5cm is highly predictive of preterm labor 1

• Fetal Echocardiography:

  • Recommended for all monochorionic twins due to 9-fold increase in cardiac anomalies 1
  • Risk increases up to 14-fold in cases of TTTS 1

• Biophysical Profile (BPP):

  • Useful in cases of growth restriction, TTTS, and other complications 1
  • No proven benefit in uncomplicated twin pregnancies 1

Clinical Implications

• Twin pregnancies have 5 times higher risk of fetal death and 7 times higher risk of neonatal death compared to singletons 1, 3
• Preterm birth occurs in 60% of twins before 37 weeks and 10.7% before 32 weeks 4
• Monochorionic twins have higher incidence of both indicated and spontaneous preterm delivery than dichorionic twins 4
• Close monitoring according to chorionicity-specific protocols is essential to improve outcomes 1, 3