The Twin Reversed Arterial Perfusion Intervention STudy (TRAPIST) is looking into whether early intervention (at 12-14 weeks gestation) improves the survival and two-year development of children compared to later intervention (at 16-18 weeks gestation) in twin pregnancies complicated by TRAP.

TRAP (twin reversed arterial perfusion) is a complication of shared circulation in monochorionic twins who share a single placenta. In TRAP cases, reversed blood flows from the healthy twin (pump twin) towards the abnormal twin. This strains the heart of the pump twin and can increase urine output, which can trigger premature birth.

The condition is now being routinely diagnosed at the 12-week routine ultrasound scan, but treatment is traditionally offered at the 16-week point.

Dr Asma Khalil at St George’s Hospital, the clinician behind the UK-TRAPIST study, says intervention before 16 weeks carries a higher risk of miscarriage. In some TRAP cases, the reversed blood flow stops by itself and the intervention planned at 16 weeks is not required.

However, it has also been shown that diagnosis at 12 weeks with treatment delayed until the 16th week is associated with a high pregnancy loss (up to 33%) of the pump twin.  Previous studies have also shown that earlier treatment usually results in delivery closer to the expected date of delivery with improved survival rates.

In order to ascertain which method is better, Dr Khalil and her team will be putting people into groups and giving each group a different treatment. The results can then be compared to see if one is better than the other.

To make sure the groups are the same to start with, each patient is randomly selected by a computer to either go in the early treatment group or the later treatment group. 

The UK-based study is part of a wider international multicentre trial led by Professor Liesbeth Lewi (Leuven, Belgium). Five centres in the UK are planning to participate. Read more here.