Twin Anaemia Polycythaemia Sequence (TAPS)

TAPS is essentially a 'slow-motion' transfusion.

• Micro-Anastomoses: In TAPS, the connections in the placenta are so small that fluid (water) cannot pass through them, but red blood cells can slowly trickle across. This is why, unlike TTTS, the amniotic fluid levels in TAPS often remain normal in both babies.
• The Anaemic Donor: Has 'thin' blood that moves very fast. This twin is at risk of heart strain and growth issues.
• The Polycythaemic Recipient: Has 'thick' blood that moves slowly, which can lead to poor circulation and potential organ damage.

TAPS can happen spontaneously or as a complication after a laser procedure for TTTS (if a tiny vessel was missed).

TAPS affects approximately 3-5% of all monochorionic twin pregnancies.

It is most often diagnosed in the later second trimester or early third trimester (after 26 weeks). It is critically important to distinguish TAPS from 'normal' twins, as the management is entirely different. Because the amniotic fluid levels look normal, TAPS is easily missed if the doctor is not routinely measuring the MCA-PSV (Middle Cerebral Artery Peak Systolic Velocity) of both twins.

The Diagnosis of TAPS is based strictly on brain Doppler (MCA-PSV) measurements:

1. Donor Twin: The blood flow in the brain is very fast (greater than 1.5 MoM), indicating anaemia.
2. Recipient Twin: The blood flow in the brain is very slow (less than 1.0 MoM), indicating thick, polycythaemic blood.
3. Placental Appearance: On ultrasound, the donor's side of the placenta may appear thickened or 'starry-sky,' while the recipient's side looks normal.

At the MFM Unit, we include MCA-PSV Doppler as a standard part of every monochorionic twin assessment from 20 weeks onwards to ensure TAPS is never overlooked.

Management options for TAPS are tailored to the severity and the weeks of pregnancy:

• Expectant Management: If the TAPS is stable and the babies are healthy, we may monitor very closely with scans every 2-3 days.
• Fetoscopic Laser Ablation: Similar to TTTS, we can use a laser to seal the tiny micro-connections. This is the only definitive cure.
• Intrauterine Transfusion (IUT): Giving the donor twin blood while they are still in the womb, and sometimes performing a partial exchange transfusion for the recipient.
• Delivery: If the pregnancy is far enough along (usually after 32 weeks), we may choose to deliver the babies and treat the anaemia/polycythaemia in the NICU.

While TAPS is a chronic condition, the outcomes are generally very good if it is detected and managed appropriately.

• Long-term studies show that most TAPS survivors do well, although there is a risk of hearing loss in some donor twins, which requires early neonatal screening.
• Unlike TTTS, TAPS rarely leads to sudden heart failure in the womb, but it can significantly impact the health of the twins at birth if they are born with extreme differences in their blood counts ('One red twin, one pale twin').

TAPS is a 'silent' condition because fluid levels remain normal. If your twin pregnancy scans do not include regular measurements of the brain blood flow (MCA-PSV), or if your doctor mentions an 'unexplained' difference in placental color, you should seek an expert MFM opinion.

Our consultants, led by Dr. Werner Diehl, are experts in the 'Leiden' protocols for TAPS and provide the specialized monitoring required to ensure these subtle changes are detected before they cause harm.