In brief — the direct answer
Twin Anemia Polycythemia Sequence (TAPS) is a form of chronic blood transfusion between identical twins sharing one placenta (MCDA). Unlike TTTS, where there is a massive fluid imbalance, TAPS is a 'slow leak' through tiny, microscopic vessels. Over time, one twin (the donor) loses red blood cells and becomes anemic (low blood count), while the other (the recipient) gains too many red blood cells and becomes polycythemic (thick blood). At the MFM Unit, we are experts in detecting this subtle but serious condition using specialized brain blood flow measurements.
What is TAPS: Twin Anemia Polycythemia Sequence?
TAPS is caused by minute vascular connections (less than 1mm in size) that allow red blood cells to pass from one twin to the other.
- The Donor (Anemic): Their blood becomes like water; they have plenty of fluid but not enough oxygen-carrying cells.
- The Recipient (Polycythemic): Their blood becomes like honey—too thick to flow easily through small vessels, which can lead to complications after birth.
- The Placenta: After birth, a TAPS placenta often shows a striking color difference—half is very pale and half is very dark red.
Who is at risk?
TAPS occurs in about 3-5% of monochorionic twin pregnancies. It can also occur in about 10% of cases after laser surgery for TTTS, if the tiny vessels at the very edge of the placenta were missed (Post-Laser TAPS).
How we diagnose it
TAPS cannot be seen by measuring amniotic fluid. It is only detected by measuring the speed of blood in the baby's brain:
- MCA-PSV Doppler: We measure the Peak Systolic Velocity in the Middle Cerebral Artery. If the blood is thin (anemic), it flows very fast. If it is thick (polycythemic), it flows slowly.
- MoM (Multiples of the Median): We compare these speeds to expected averages for their gestational age. A wide gap between the two twins confirms TAPS.
- Placental Appearance: Sometimes we see a 'starry sky' appearance of the liver in the polycythemic twin on ultrasound.
What does management involve?
Treatment depends on the severity and how far along the pregnancy is:
- Expectant Management: Close monitoring (weekly) if the levels are stable and the babies are near a safe delivery age.
- Intrauterine Transfusion (IUT): Giving the anemic baby a blood transfusion while still in the womb.
- Laser Surgery: Using a laser to block the tiny vessels, similar to the treatment for TTTS.
- Early Delivery: If the anemia becomes critical, we may deliver the twins early to allow the neonatal team to treat them directly.
What are the outcomes?
The long-term outlook for TAPS survivors is generally very good:
- Post-Birth Treatment: The polycythemic twin may need a 'partial exchange transfusion' (replacing some thick blood with saline) and the anemic twin may need a blood transfusion.
- Follow-up: We recommend specialized hearing tests for TAPS survivors, as high red blood cell counts in the womb have been linked to a slightly higher risk of mild hearing issues.
When to see a subspecialist
Arrange fetal-medicine review when TAPS is suspected or MCA Doppler results differ substantially between monochorionic twins. Assessment considers the diagnostic criteria, stage and gestation. Routine surveillance remains important after laser treatment for TTTS.