Twin anemia polycythemia sequence (TAPS) is a rare complication of monochorionic twin pregnancies where chronic, slow blood transfer through tiny placental vessels causes one twin to become severely anemic while the other develops dangerously thick, polycythemic blood. It affects roughly 3–5% of uncomplicated monochorionic-diamniotic (MCDA) twin pregnancies spontaneously, and up to 2–13% of pregnancies after laser surgery for twin-to-twin transfusion syndrome (TTTS).
Unlike classic TTTS — which involves large-volume fluid shifts causing one twin to have too much amniotic fluid and the other too little — TAPS is sneakier. The placental connections are tiny (often <1 mm), the blood transfer is slow, and amniotic fluid levels usually stay normal. That’s what makes it easy to miss and dangerous when it’s caught late.
How TAPS Differs from Classic TTTS
This distinction matters clinically because TAPS requires different screening, different diagnostic criteria, and sometimes different treatment. Here’s a quick comparison:
| Feature | TTTS | TAPS |
|---|---|---|
| Placental anastomoses | Large arteriovenous connections | Very small (<1 mm) connections |
| Transfer type | Rapid fluid/volume shifts | Slow, chronic red blood cell transfer |
| Amniotic fluid | Polyhydramnios/oligohydramnios (donor-recipient) | Usually normal in both twins |
| Onset | Typically 16–26 weeks | Can occur anytime; often later in pregnancy |
| Key diagnostic tool | Amniotic fluid assessment | MCA-PSV Doppler |
| Hemoglobin difference at birth | Variable | ≥8 g/dL between twins |
What Causes TAPS?
TAPS occurs because monochorionic twins share a single placenta, and in some cases, minuscule arteriovenous anastomoses allow a one-directional trickle of red blood cells from the “donor” twin to the “recipient.” Over weeks to months, the donor becomes progressively anemic while the recipient accumulates excess red blood cells.
There are two forms:
- Spontaneous TAPS: Develops on its own in ~3–5% of MCDA pregnancies, typically detected after 26 weeks.
- Post-laser TAPS: Occurs in 2–13% of cases after fetoscopic laser ablation for TTTS, when small anastomoses are missed during the procedure.
IVF pregnancies carry a slightly higher rate of monochorionic twinning, which indirectly raises TAPS risk — but TAPS itself is a consequence of placental architecture, not conception method.
Prenatal Diagnosis: The MCA-PSV Criteria
The gold standard for detecting TAPS before birth is middle cerebral artery peak systolic velocity (MCA-PSV) measured by Doppler ultrasound. When a fetus is anemic, its blood flows faster to compensate — so MCA-PSV rises. When polycythemic, the thickened blood flows slower.
The Leiden University Medical Center classification, widely used in clinical practice, defines prenatal TAPS as:
- Donor twin: MCA-PSV >1.5 multiples of the median (MoM) — indicating anemia
- Recipient twin: MCA-PSV <1.0 MoM — indicating polycythemia
Both criteria should be met simultaneously. Current guidelines recommend MCA-PSV screening every 2 weeks starting at 16 weeks in all MCDA pregnancies, and biweekly after laser surgery for TTTS.
TAPS Staging System
TAPS is staged both prenatally (antenatal) and after delivery (postnatal):
| Stage | Antenatal Criteria (MCA-PSV) | Postnatal Criteria |
|---|---|---|
| 1 | Donor >1.5 MoM; Recipient <1.0 MoM | Inter-twin Hb difference >8 g/dL |
| 2 | Donor >1.7 MoM; Recipient <0.8 MoM | Hb difference >8 g/dL + reticulocyte ratio >1.7 |
| 3 | Stage 1 or 2 + critically abnormal Dopplers (absent/reversed end-diastolic flow in umbilical artery, pulsatile umbilical venous flow, or increased pulsatility index in ductus venosus) | Hb difference >8 g/dL + reticulocyte ratio >1.7 + one donor reticulocyte count >percentage threshold |
| 4 | Hydrops in either twin | — |
| 5 | Intrauterine demise of one or both twins | — |
Treatment Options
Management depends on gestational age, staging, and available expertise. There’s no single consensus protocol, but options include:
- Expectant management with close surveillance: For stage 1 TAPS, many centers opt for biweekly MCA-PSV monitoring, since some cases stabilize or even resolve spontaneously.
- Intrauterine transfusion (IUT): Transfusing the anemic donor can buy time, but it’s a temporizing measure — the underlying placental connections remain.
- Fetoscopic laser ablation: The most definitive prenatal treatment. A fetoscope identifies and coagulates the tiny anastomoses. This is technically challenging because TAPS vessels are much smaller than those in TTTS.
- Early delivery: When TAPS is diagnosed late (beyond 30–32 weeks) or progresses rapidly, delivery with neonatal management may be the safest option.
- Selective fetal reduction: Rarely considered, and only in severe cases where one twin’s condition threatens the other’s survival.
Outcomes and Survival Rates
Outcomes vary significantly by stage and timing of diagnosis. Overall perinatal survival in treated TAPS is approximately 80–90% for at least one twin and roughly 70% for both twins surviving, though these numbers come from relatively small case series at specialized centers.
The anemic donor twin faces the greatest risk of severe neonatal morbidity, including the need for immediate transfusion at birth. The polycythemic recipient can develop hyperviscosity syndrome, which may cause thrombosis, jaundice, and organ damage if not treated promptly with partial exchange transfusion after delivery.
Long-term neurodevelopmental outcomes are still being studied, but early data suggest that stage 3+ TAPS and cases involving intrauterine fetal demise of one twin carry the highest risk of brain injury in the surviving co-twin.
Postnatal Signs to Watch For
At delivery, the clinical picture is often dramatic:
- The donor twin appears pale, may be growth-restricted, and has hemoglobin levels that can drop below 8 g/dL
- The recipient twin appears plethoric (deep red), and hemoglobin can exceed 20 g/dL or even 25+ g/dL
- A hallmark finding is a “starry sky” placental appearance — the donor’s share looks pale and the recipient’s share looks engorged and dark
Frequently Asked Questions
Can TAPS resolve on its own?
Stage 1 TAPS can sometimes stabilize or even regress without intervention. However, it can also progress unpredictably, which is why continued biweekly MCA-PSV monitoring is essential even if things initially look mild.
How is TAPS different from regular twin-to-twin transfusion syndrome?
The key difference is the size of placental connections and what gets transferred. TTTS involves large anastomoses causing rapid fluid shifts with amniotic fluid imbalances. TAPS involves tiny anastomoses causing slow red blood cell transfer with hemoglobin discordance but typically normal amniotic fluid.
What hemoglobin difference confirms TAPS after birth?
An inter-twin hemoglobin difference of >8 g/dL is the primary postnatal diagnostic threshold. A reticulocyte count ratio >1.7 between the twins supports the diagnosis and helps with staging.
Is TAPS always detected during pregnancy?
No. TAPS is missed in a significant number of cases, especially when routine MCA-PSV screening isn’t performed. Some studies suggest that up to 5% of MCDA pregnancies have undiagnosed TAPS discovered only after delivery when one twin is unexpectedly anemic and the other polycythemic.
Which hospitals specialize in TAPS treatment?
TAPS management requires a fetal therapy center with experience in fetoscopic laser surgery. In the U.S., major programs include those at Cincinnati Children’s, Texas Children’s, Children’s Hospital of Philadelphia (CHOP), and Brigham and Women’s Hospital. In Europe, Leiden University Medical Center in the Netherlands is a leading TAPS research and treatment center.
Key Takeaways
- TAPS is a chronic, slow form of blood transfer between monochorionic twins — distinct from classic TTTS
- Biweekly MCA-PSV Doppler screening starting at 16 weeks is critical for early detection
- Prenatal diagnosis requires MCA-PSV >1.5 MoM in the donor AND <1.0 MoM in the recipient
- Postnatal diagnosis requires an inter-twin hemoglobin difference >8 g/dL
- Treatment ranges from watchful waiting (stage 1) to laser ablation or early delivery
- If you’re carrying MCDA twins, ask your MFM specialist whether routine MCA-PSV screening is part of your monitoring plan — early detection dramatically improves outcomes
