Death of One Twin During Pregnancy (Single Intrauterine Fetal Demise - sIUFD)

1. Terminology

2. How Common Is It?

• Vanishing twin in the first trimester: reported in ~21% of twin pregnancies
• sIUFD after 12 weeks: occurs in ~6% of all twin pregnancies overall
• Before 22 weeks: ~0.7% of dichorionic and ~0.9% of monochorionic-diamniotic twin pregnancies

3. Causes (Pathogenesis)

• Fetal causes: discordant infections, discordant structural anomalies, discordant fetal growth, abnormal fetoplacental blood flow
• Placental causes: uneven placental sharing, placental implantation anomalies, peripheral cord insertion
• Maternal causes: preeclampsia, thrombophilia, placental abruption
• Iatrogenic: after selective feticide, after laser therapy for twin-to-twin transfusion syndrome (TTTS)

4. Why Chorionicity is the Single Most Important Factor

• Blood acutely shifts from the surviving twin into the dead twin through these connections
• This causes sudden, severe hypotension and acute anemia in the survivor
• This is the primary mechanism of neurological injury - it happens at the moment of death, so obstetric intervention is often already too late to prevent it

5. How to Identify / Diagnosis

First Trimester (Vanishing Twin)

• Ultrasound is the primary tool - a second gestational sac or embryo is seen early but absent on follow-up scan
• A collapsed/partially resorbed sac with embryonal remnants (vertebral column, ocular pigment) may be found
• Genetic evidence of a resorbed twin may appear as restricted placental chimerism

Second/Third Trimester

• Ultrasound: absence of fetal cardiac activity in one fetus; may show echogenic (compressed/macerated) fetal parts - fetus papyraceus
• MRI of the fetal brain: superior to ultrasound for detecting neurological injury in the surviving twin - detects brain abnormalities in 13% of monochorionic survivors; ultrasound under-calls severity in ~67% of these cases
• Neurosonography (fetal brain ultrasound): first-line imaging for the survivor's brain
• Look for: multicystic encephalomalacia, white matter lesions, cortical changes
• Maternal coagulation screen: disseminated intravascular coagulation (DIC) was historically feared (once estimated 25% risk), but modern data show clinical DIC is rare after spontaneous or selectively terminated twin demise

Placental pathology (post-delivery)

• Focal membrane thickening on the chorionic plate of an apparently singleton placenta
• Plaque-like thickening on one side of the placental disk (fetal side)

Fig: Diamniotic-dichorionic twin placenta with vanishing twin syndrome. (A) Apparently singleton placenta with focal membrane thickening. (B) Closer view showing separate sac with macerated, malformed embryo. - Creasy & Resnik's Maternal-Fetal Medicine

6. Risks to the Surviving Twin

Neurological Injury

• Risk of neurological injury at any gestational age in monochorionic twins - there is no safe early gestational cutoff; injury has been reported after co-twin death at as early as 12 weeks
• Risk is highest when sIUFD occurs between 28-33 weeks gestation in monochorionic pregnancies
• Surviving fetuses from vanishing twin pregnancies have increased risk of small for gestational age (inversely related to gestational age at time of co-twin death)

Preterm Birth

• Increased risk of medically indicated or spontaneous preterm birth

7. Management

General Principles

First Trimester Vanishing Twin

• Usually requires no specific intervention
• Continue routine antenatal surveillance
• Be aware of increased SGA risk in the surviving twin

Second/Third Trimester sIUFD

Monochorionic Twins

• Neurological injury likely already occurred at moment of death - emergent delivery rarely helps the surviving twin's brain
• Fetal brain neuroimaging (neurosonography + fetal MRI) to assess the surviving twin
• If fetal demise has already occurred: close surveillance with serial ultrasound; delivery at 37 weeks is reasonable
• If one fetus appears premorbid (about to die): the 26% risk of profound neurological injury after co-twin death must be weighed against complications of prematurity - delivery may be considered depending on gestational age

Dichorionic Twins

• Risk to surviving twin is much lower (~2-3% co-death, 2% serious neurological injury)
• Expectant management is generally appropriate
• Serial surveillance with ultrasound
• Monitor maternal coagulation profile (DIC, although rare, is possible with prolonged retention)
• Delivery timing should balance gestational age against ongoing risks

Delayed-Interval Delivery (Special scenario: one twin delivers preterm)

• Protocol: cord ligation of the delivered twin near the placenta, tocolysis, broad-spectrum antibiotics, anti-D prophylaxis (if Rh-negative), possible cerclage placement, serial amniocentesis to exclude infection
• Mean latency achieved: ~36 days (range 3-123 days)
• Survival rates for the remaining twin: 50% when initial delivery before 25 weeks
• Meta-analysis of 492 cases: delayed delivery significantly improved survival (OR 5.22; 95% CI 2.95-9.25)
• Serious maternal morbidity reported in 39% of cases with this approach - careful counseling is mandatory

Maternal Monitoring

• Maternal coagulation profile (PT, aPTT, fibrinogen, D-dimer) - especially important if demise is prolonged or suspected
• Anti-D immunoglobulin in Rh-negative mothers
• Grief counseling and psychological support

8. Summary Table