Zygotic twin anemia polycythemia sequence (ZAPSS) - Symptoms, Causes, Treatment & Prevention

📅 Updated: June 2026 ⏱️ 6 min read ✅ Medically reviewed
```html Zygotic Twin Anemia‑Polycythemia Sequence (ZAPSS) – Complete Guide

Zygotic Twin Anemia‑Polycythemia Sequence (ZAPSS)

Overview

Zygotic Twin Anemia‑Polycythemia Sequence (ZAPSS) is a rare and serious complication that can occur in monochorionic (shared‑placenta) twin pregnancies. The condition results from an uneven blood‑flow shunting through vascular connections (anastomoses) in the placenta, causing one twin to become severely anemic while the other becomes polycythemic (excess red blood cells). Because the twins share a single placenta, the blood loss from the anemic twin is transferred directly to the other twin. ZAPSS most commonly affects identical (monozygotic) twins that share a placenta (monochorionic‑diamniotic or monochorionic‑monoamniotic). It is distinct from Twin‑to‑Twin Transfusion Syndrome (TTTS), which involves both twins becoming polycythemic; in ZAPSS the imbalance is more extreme, with one side anemia.

Prevalence: Precise incidence is difficult to determine because many cases are missed or misdiagnosed, but estimates from tertiary care centers place ZAPSS at approximately 1–2 per 10,000 monochorionic twin pregnancies (Cunningham et al., 2022). With improved ultrasound screening, the condition is being recognized more frequently.

Symptoms

Symptoms can develop in utero, manifest after birth, or evolve during the neonatal period. The clinical picture differs between the anemic and polycythemic twin.

Anemic Twin (Twin A)

  • Fetal growth restriction (FGR): slower growth on serial ultrasounds.
  • Non‑reassuring fetal heart rate patterns: decelerations, reduced variability.
  • Hydrops fetalis: abnormal fluid accumulation in skin, abdomen, or lungs.
  • Low amniotic fluid (oligohydramnios): due to decreased urine output.
  • Post‑natal pallor, lethargy, & tachycardia: signs of anemia after delivery.

Polycythemic Twin (Twin B)

  • Elevated hematocrit (>65%): best detected by blood tests.
  • Respiratory distress: due to increased blood viscosity and reduced oxygen delivery.
  • Hyperbilirubinemia: jaundice that may develop within the first 24‑48 hours.
  • Neurologic irritability: seizures or abnormal movements from hyperviscosity.
  • Cardiac overload: tachycardia, cardiomegaly on echocardiogram.

Maternal Signs

  • Sudden increase in uterine size (due to polyhydramnios from the polycythemic twin).
  • Unexplained abdominal pain or pre‑term labor.

Causes and Risk Factors

ZAPSS is driven by an imbalance in placental vascular connections. The exact cause of the imbalance is not fully understood, but several factors increase the likelihood:

  • Monochorionic placentation: twins sharing a single placenta (≈20 % of all twin pregnancies).
  • Large superficial arterio‑arterial (AA) or arterio‑venous (AV) anastomoses: allow rapid, unregulated blood flow from one twin to the other.
  • Early gestational age at detection: the earlier the vascular imbalance, the greater the cumulative effect.
  • Previous twin complications: a history of TTTS, selective intrauterine growth restriction (sIUGR), or placental abnormalities.
  • Maternal factors: advanced maternal age (>35 years), assisted reproductive technologies (IVF), and pre‑eclampsia have been associated with higher rates of placental vascular anomalies.

Diagnosis

Accurate diagnosis requires a combination of detailed ultrasound assessment, Doppler studies, and laboratory testing.

Prenatal Imaging

  • Serial ultrasounds: measure bi‑parietal diameter, abdominal circumference, and estimated fetal weight for each twin.
  • Middle cerebral artery (MCA) Doppler: increased peak systolic velocity in the anemic twin indicates low viscosity; decreased velocity in the polycythemic twin suggests high viscosity.
  • Placental mapping: colour Doppler identifies large AA or AV anastomoses.
  • Fetal echocardiography: assesses cardiac strain in the polycythemic twin.

Post‑natal Evaluation

  • Complete blood count (CBC): hematocrit < 40 % in the anemic twin, >65 % in the polycythemic twin.
  • Blood gas analysis: evaluates oxygenation and acid‑base status.
  • Transfontanelle ultrasound: screens for intracranial hemorrhage in the polycythemic twin.
  • Serum bilirubin and reticulocyte count: monitors hemolysis and hyperbilirubinemia.

Diagnostic Criteria (Adapted from International Consensus, 2021)

  1. Monochorionic twin pregnancy confirmed by ultrasound.
  2. Discordant MCA Doppler velocities > 2‑standard deviations between twins.
  3. Post‑natal hematocrit difference ≥ 30 % (e.g., 35 % vs. 70 %).
  4. Absence of classic TTTS criteria (i.e., no bladder filling disparity).

Treatment Options

Management is individualized, based on gestational age, severity of anemia/polycythemia, and fetal condition.

In‑utero Interventions

  • Selective Fetoscopic Laser Photocoagulation (SFLP): laser ablation of the dominant AA/AV anastomoses to stop the unbalanced shunt. Evidence shows a 65–80 % success rate in halting progression when performed before 26 weeks (Cunningham et al., 2022).
  • Intra‑uterine Blood Transfusion (IUT): Reserved for severe anemia (< 30 % hematocrit) in the donor twin. Performed via cordocentesis; carries a 5–10 % risk of fetal loss.
  • Amnioreduction or Thoraco‑Amniotic Shunting: May be used if polyhydramnios contributes to maternal discomfort or pre‑term labor.

Delivery Planning

  • Timing: Early delivery (34‑36 weeks) is often recommended once fetal lung maturity is confirmed, to avoid further deterioration.
  • Mode: Cesarean section is preferred when one twin is critically ill, to allow rapid neonatal resuscitation.

Neonatal Management

  • For the anemic twin:
    • Partial exchange transfusion or simple packed‑red‑cell transfusion to raise hematocrit to 45–55 %.
    • Supportive care: thermoregulation, glucose monitoring, and treatment of any co‑existing infections.
  • For the polycythemic twin:
    • Partial exchange transfusion (PEET) with normal saline to reduce hematocrit < 65 % and improve viscosity.
    • Phototherapy for hyperbilirubinemia; consider intensive phototherapy if bilirubin > 20 mg/dL.
    • Hydration and diuretics if pulmonary edema develops.

Long‑Term Follow‑up

  • Neurodevelopmental assessments at 6 months, 1 year, and annually thereafter.
  • Echocardiograms for the polycythemic twin to monitor for pulmonary hypertension.
  • Hematology clinic visits for any delayed anemia or polycythemia.

Living with Zygotic Twin Anemia‑Polycythemia Sequence (ZAPSS)

Families face a steep learning curve after a ZAPSS diagnosis. Below are practical tips to aid daily life.

  • Coordinate a multidisciplinary team: obstetrician‑maternal‑fetal medicine specialist, neonatologist, pediatric hematologist, and a developmental therapist.
  • Maintain a health log: record feeding volumes, urine output, bilirubin levels, and any changes in activity.
  • Feeding: Breast‑milk is preferred; if using formula, ensure adequate caloric density to support catch‑up growth.
  • Vaccinations: Follow standard immunization schedule; note that transfused infants may need delayed live vaccines (e.g., rotavirus) per CDC guidelines.
  • Environmental safety: Keep the home temperature stable (36–37 °C for newborns) and avoid exposure to tobacco smoke, which can worsen pulmonary hypertension.
  • Parental support: Seek counseling or support groups (e.g., Twins Foundation) to address anxiety and caregiver fatigue.

Prevention

Because ZAPSS is intrinsically linked to placental vascular anatomy, true primary prevention is limited. However, certain measures can reduce risk or facilitate early detection:

  • Early and accurate chorionicity assessment: At the first‑trimester scan (7‑13 weeks) confirm monochorionic status.
  • Routine specialized ultrasound for monochorionic twins: Serial growth scans and MCA Dopplers every 2‑3 weeks from 16 weeks onward.
  • Maternal health optimization: Control hypertension, avoid smoking, and manage diabetes to reduce placental abnormalities.
  • Consideration of IVF protocols: When possible, single embryo transfer for patients at high risk of multiple gestations.

Complications

If untreated or inadequately managed, ZAPSS can lead to serious short‑ and long‑term complications:

  • Severe fetal anemia: leading to hydrops, intra‑uterine death, or pre‑term delivery.
  • Hyperviscosity syndrome in the polycythemic twin: causing stroke, intracranial hemorrhage, or necrotizing enterocolitis.
  • Chronic lung disease: from early‑preterm birth and pulmonary hypertension.
  • Neurodevelopmental delays: cognitive, motor, or visual impairment reported in up to 30 % of affected twins (Cunningham et al., 2022).
  • Cardiac sequelae: ventricular hypertrophy or persistent pulmonary hypertension of the newborn (PPHN).

When to Seek Emergency Care

Call 911 or go to the nearest emergency department if your newborn (or any twin) shows any of the following signs:
  • Rapid, irregular, or very weak breathing (respiratory distress).
  • Blue‑tinged lips or skin (cyanosis) that does not improve with crying.
  • Severe lethargy or inability to wake for feedings.
  • Persistent high‑grade fever (> 38.5 °C / 101.3 °F) in the first week of life.
  • Sudden swelling of the head, abdomen, or limbs (possible hydrops).
  • Visible seizure activity – jerking, staring, or limpness.
  • Rapid heart rate (> 200 bpm) or very low heart rate (< 80 bpm) while the baby is at rest.

These symptoms may reflect worsening hyperviscosity, severe anemia, or cardiac compromise and require immediate evaluation.

**References**

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Important: The information provided on this page is for general informational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

If you think you may have a medical emergency, call your doctor, go to the emergency department, or call 911 immediately.

📚 Medical References