What is TTTS?
TTTS is a very serious complication, which is only seen with monochorionic twins (who share a placenta). The risk of TTTS developing in a monochorionic-diamniotic twin pregnancy is 10-15%. It is now known that TTTS includes “classic TTTS” and a related condition, called TAPS (see below).
The condition is due to an unequal sharing of blood supply in the placenta. Although MCDA twins share a single placenta, they each “own” their own half. In TTTS, small blood vessels which connect the 2 halves of the placenta allow uneven blood flow, such that one twin (the “donor”) does not get enough blood and the other twin (the “recipient”) gets overloaded with fluid. TTTS is not a risk in dichorionic twins, as each twin has their own individual placenta.
How is TTTS diagnosed?
TTTS can only be diagnosed on ultrasound scan. The first sign is a difference in the quantity of amniotic fluid in the 2 sacs. As the condition progresses, abnormalities in the fetal bladders and placental blood flow may be detected.
Given the seriousness of TTTS, all women with MCDA twins require regular specialist ultrasound scans (every 2 weeks), beginning at 16 weeks and continuing for the entire pregnancy. TTTS is the reason why monochorionic twin pregnancies are considered higher risk than dichorionic twin pregnancies.
Can TTTS be treated?
If TTTS is diagnosed promptly, treatment can be offered. The best treatment depends on the degree of severity of the condition and the gestational age. The severity of TTTS is described using the Quintero staging system:
What is TAPS?
TAPS is a complication of twins which has only been properly identified in recent years. TAPS is related to classic twin-twin transfusion syndrome (TTTS) and, like TTTS, it only occurs in monochorionic twin pregnancies. TAPS occurs in up to 5% of MCDA twins.
Like classic TTTS, TAPS is due to uneven blood flow between the two halves of a shared twin placenta. As a result, one twin (the “donor”) loses blood and becomes pale and anaemic while the co-twin (the “recipient”) receives too much blood and becomes ruddy in colour and polycythaemic. Polycythaemia means there are too many red blood cells in the circulation (it is the opposite of anaemia).
How is TAPS diagnosed?
Like classic TTTS, TAPS is usually diagnosed on antenatal ultrasound scan. Occasionally, TAPS is only diagnosed after delivery, if one twin looks very pale and the other is dark red. Unlike TTTS, twins with TAPS do not have abnormal amniotic fluid levels on ultrasound. The diagnosis can only be made if a special test of the blood flow in the baby’s brain – called the middle cerebral artery Doppler – is performed. Fetuses who are anaemic have fast blood flow in the brain and fetuses who are polycythaemic have slow flow.
It is now recommended that, after 20 weeks, ultrasound scans of MCDA twins should always include MCA Doppler testing. However, often this test is only offered in specialised fetal medicine centres, which means that many cases of TAPS go unrecognised.
Can TAPS be treated?
Because TAPS has only recently been described, the best treatment is not yet fully understood. Unlike classic TTTS, laser therapy is not usually offered, as the abnormal vessels joining the two halves of the placenta are tiny and very hard to identify. The most common treatments for TAPS are either a “wait-and-see” approach with regular ultrasound scans or emergency delivery. This is a complex decision, which depends on the gestational age and other factors, and should only be made by an experienced fetal medicine specialist. Occasionally, TAPS may be treated by an “in utero transfusion” of blood to the donor twin, to correct the anaemia.
Further information on TAPS is available at www.tapsregistry.org.