What is a vanishing twin (multiple gestations disappearance in the uterus during pregnancy)?

Vanishing Twin Syndrome: Diagnosis and Implications

A vanishing twin refers to the spontaneous demise of one fetus in a multiple gestation pregnancy during the first trimester, with subsequent resorption of the embryonic tissue, resulting in a singleton pregnancy at delivery. 1

Definition and Prevalence

• Vanishing twin syndrome (VTS) occurs when one embryo in a multiple gestation pregnancy spontaneously disappears during early pregnancy
• Prevalence ranges from 15-35% of twin pregnancies identified in the first trimester 2
• The phenomenon is primarily detected through ultrasound examination when a previously identified gestational sac or embryo is no longer visible on subsequent scans

Timing and Mechanism

• Most vanishing twin events occur in the first trimester, typically before 10 weeks gestation 3
• Three main physiological mechanisms of disappearance:
  • Complete resorption of embryonic tissue (most common in early losses)
  • Formation of a blighted ovum
  • Development of fetus papyraceus (compressed fetal remains) when loss occurs later in pregnancy 3, 4

Risk Factors

• Advanced maternal age
• Increased number of embryos in multiple gestations
• In IVF pregnancies: higher number of embryos transferred 2
• The risk is higher in monochorionic twins due to their shared placental circulation and higher rate of complications 5

Clinical Presentation and Diagnosis

• Most cases are asymptomatic and discovered incidentally during routine ultrasound
• When symptomatic, the most common presentation is first-trimester vaginal bleeding with or without abdominal cramping 4
• Diagnosis is made through serial ultrasound examinations showing:
  • Initial identification of multiple gestational sacs/embryos
  • Subsequent disappearance of one or more embryos on follow-up ultrasound
  • Continued development of the surviving fetus(es) 1

Clinical Implications

For the Surviving Fetus

• Controversial impact on perinatal outcomes:
  • Some studies show increased risk for preterm birth and small-for-gestational-age neonates compared to singleton pregnancies
  • Other studies demonstrate no significant difference in outcomes 2
• Higher risk of placental vascular and anatomic abnormalities in pregnancies with VTS 2

For Prenatal Testing

• VTS can affect the accuracy of noninvasive prenatal testing (NIPT)
• Cell-free DNA from the vanished twin may persist in maternal circulation, potentially causing false-positive results 6
• NIPT has shown to be effective for screening trisomy 21 in VTS pregnancies (positive predictive value of 50%), but results for trisomies 18 and 13 should be interpreted with caution 6

Management Considerations

• Caution is warranted when establishing viability of twins in early pregnancy due to the relatively common occurrence of vanishing twin 1
• When first-trimester bleeding occurs with ultrasound evidence of multiple gestations:
  • Avoid therapeutic dilation and curettage until a follow-up sonogram confirms the status of all fetuses 4
  • Continue monitoring with serial ultrasounds to assess the surviving fetus(es)
• Documentation of vanishing twin is important as it affects interpretation of subsequent prenatal screening tests and may influence pregnancy management 7

Key Pitfalls to Avoid

• Misdiagnosing threatened abortion in a singleton pregnancy when it's actually a vanishing twin event
• Performing unnecessary interventions based on first-trimester bleeding without complete ultrasound assessment
• Failing to document VTS, which can lead to misinterpretation of prenatal screening results
• Not considering the potential impact of VTS on the surviving pregnancy when planning prenatal care

Follow-up Recommendations

• For pregnancies with documented VTS:
  • Continue routine prenatal care with appropriate monitoring
  • Consider increased surveillance for growth restriction in the surviving fetus
  • Interpret prenatal screening results with caution, particularly NIPT for trisomies 18 and 13
  • Be aware of the potential for placental abnormalities at delivery