Monochorionic Twins

illustration of monochromic twins - ©chrisgralapp.commedical

What are monochorionic twins?

A single placenta normally supports a single fetus. When the situation arises in which two fetuses have to share a single placenta, complications may sometimes develop. Identical twins that share a single placenta are called monochorionic twins (MC). “Chorion” is the Latin root that refers to the placenta, while the word “amnion” refers to the sac, or “membranes” that surround each fetus. While fraternal twins (2 eggs and 2 sperm) are always surrounded in their own sacs and have their own individual placentas, 70% of identical twins may end up sharing a single placenta. Only 1% of identical twins share both a single placenta and a single sac, and this poses significant risk.

When two fetuses share one placenta, their umbilical cords may implant anywhere – there is no set or predictable pattern – and depending on where they implant, one fetus may get less of a ‘share’ of the placenta than it’s co-twin, resulting in less blood flow and nutrition to one fetus, with more to the other (unequal placental sharing). As a result, although identical twins usually share the same genetic material, they may actually grow differently. Like the roots of a tree, the blood vessels that run from each implanted cord may connect with each other beneath the surface, as there is nothing separating them within a single placenta. Depending on which types of vessels connect to which, one fetus may transfuse blood to the other. We will discuss each of these complications, their risks, and potential treatments, below.

Monochorionic Twins Recommendations

The following recommendations are meant for both patients and their providers as guidance during a pregnancy with monochorionic twins. For questions and referrals please call us at 1-800-RX-FETUS.

You can also download and print this information with our Monochorionic Twins Recommendations PDF brochure.

If insurance authorization is required, please download the Fetal Surgery FTC Codes.

Complications Unique to Monochorionic Twins

  • Amniotic fluid discordance
  • Growth discordance ( > 20%)
  • Unequal placental sharing
  • Selective intrauterine growth restriction (S-IUGR) in one fetus
  • TTTS - clinically defined as a deepest vertical pocket of > 8 cm in one twin and < 2 cm in in the other twin, simultaneously. Note: growth discordance may also be seen but not necessary for the diagnosis.
  • Anomalies in on fetus
  • TRAP Sequence

Components of UCSF Evaluation

  • Level II anatomic survey for fetal anomalies
  • Special attention paid to cord insertions and vascular mapping
  • Fetal echocardiagraphy both for the structural integrity and functional (systolic and diastolic) pathology
  • Fetal brain MRI > 22 weeks when indicated and appropriate

Potential Surgical Procedures

  • Selective fetoscopic laser ablation
  • Radiofrequency ablation (RFA) cord occlusion

Timeframe for Referrals to UCSF

Consider Referral

  • Significant amniotic fluid discordance
  • While TTTS is defined as having a DVP > 8 cm and < 2 cm simultaneously, we encourage referral for evaluation if the fluid pockets become significantly discordant even before TTTS criteria are met
  • Significant growth discordance defined as > 20% difference
  • Polyhydramnios in one twin with normal fluid in the other
  • Suspicion of discordant anomaly

We encourage you to call our center to discuss your findings if you have concerns at 1-800-RX-FETUS

Call for Timely Referral (need to see patient within 1 week)

  • DVP > 8 cm and < 2 cm with bladder visualized in donor and normal umbilical artery dopplers

Call for Urgent Referral (need to see patient within a couple days)

  • DVP > 8 cm and < 2 cm with no visible bladder in donor and/or abnormal umbilical doppler
  • Suspected Hydrops

What is Needed For Referral

Patients referred to UCSF for an evaluation should have the following faxed to 415-502-0660: