What twin-twin transfusion syndrome is
Twin-twin transfusion syndrome (TTTS) is a serious complication that arises in monochorionic twin pregnancies — that is, twin pregnancies in which both fetuses share a single placenta. In monochorionic pregnancies the placenta contains vascular anastomoses connecting the two fetal circulations. When blood flows preferentially from one twin to the other across these anastomoses without adequate return, a chronic transfusion imbalance develops. The donor twin becomes progressively anaemic, growth- restricted and oliguric; the recipient twin becomes polycythaemic, volume-overloaded and at risk of hydrops and cardiac dysfunction.
TTTS affects approximately 10 to 15 percent of monochorionic twin pregnancies. Untreated, the condition carries a perinatal mortality of 60 to 100 percent. Even with intervention — principally fetoscopic laser ablation of the anastomoses — the combined survival rate for both twins is approximately 50 to 70 percent in specialist centres, and neurological sequelae are common in survivors. TTTS is therefore one of the leading causes of perinatal death in monochorionic-twin pregnancies, and its sequelae extend into the neonatal period even when fetoscopic intervention has taken place.
Why monochorionic-twin pathology matters
The clinical significance of a monochorionic twin pregnancy is that both fetuses share the same placental environment. Any placental compromise — whether arising from TTTS, selective intrauterine growth restriction, or placental insufficiency more broadly — affects both twins simultaneously, though not necessarily with identical presentation or timing. The donor twin typically presents with the more acute haematological compromise, while the recipient twin carries the higher risk of cardiac dysfunction, polycythaemia-related thrombosis, and hydrops.
This shared-pathology architecture means that sequential deteriorations in monochorionic twins — where one twin deteriorates, stabilises or dies, and the other then deteriorates — are the expected clinical trajectory when underlying placental pathology is present. The sequence does not require a common external cause. It is mechanistically explained by the shared vascular environment and the haemodynamic consequences of a single co-twin’s death or deterioration on the surviving twin’s circulation.
When a monochorionic co-twin dies in utero or shortly after birth, the surviving twin is at high risk of acute anaemia, hypotension and neurological injury due to the sudden shift in the shared vascular pressure gradient. This effect is well-documented in the neonatal literature and represents an important natural-cause explanation for postnatal deterioration in the surviving twin of a monochorionic pair.
What the antenatal record shows for Babies A and B
Independent paediatric review, including commentary by Dr Martyn Pitman, has examined the antenatal Doppler trajectory for Babies A and B. Antenatal Doppler studies assess blood-flow waveforms in the umbilical artery and middle cerebral artery and are the primary surveillance tool for detecting haemodynamic compromise in twin pregnancies at risk of TTTS or growth restriction.
The antenatal record is reported to show Doppler abnormalities consistent with developing placental compromise in the period before delivery. Supporters of a Criminal Cases Review Commission referral note that this trajectory was documented contemporaneously and was available to clinicians managing the pregnancy. The clinical team was therefore aware of the risk profile before delivery occurred.
The Panel’s case-by-case review examined the antenatal Doppler findings alongside the postnatal clinical course and identified a consistent pattern: the deteriorations observed after delivery were on a continuum with the antenatal compromise, rather than representing a discrete postnatal event requiring a non-natural explanation.
Why sequential deteriorations are not anomalous
The Crown alleged that the sequence of deteriorations in Babies A and B was inconsistent with natural causes and pointed to Letby’s presence as the common factor. Supporters of a CCRC referral contend that the Crown’s analysis failed to account for the expected clinical behaviour of monochorionic twins with documented placental pathology.
In such pregnancies, sequential deteriorations are the norm rather than the exception. Neonatal literature describes co-twin death as an independent risk factor for cardiovascular collapse in the surviving twin. The sequence — deterioration, partial stabilisation, further deterioration — maps onto the haemodynamic pattern expected when a monochorionic twin survives an acute co-twin haemodynamic event with residual circulatory compromise. No external intervention is required to explain this trajectory in a neonate whose antenatal record already shows placental vascular abnormality.
The prosecution’s shift-chart argument, identifying Letby’s presence at each deterioration, does not address this mechanistic context. The presence of a nurse at a deterioration in a critically ill monochorionic twin is expected: critically ill infants require continuous nursing attention, and correlation of attendance with deterioration is inevitable in that clinical environment.
What the Panel concluded
The independent review panel found natural-cause mechanisms consistent with monochorionic- twin placental pathology for the deteriorations in Babies A and B. The Panel’s conclusions do not require speculation about undetected TTTS: the documented antenatal Doppler trajectory provides the evidential foundation, and the postnatal clinical course follows from it as a matter of neonatal physiology.
Dr Martyn Pitman’s review of the Doppler findings is reported to support the conclusion that the placental vascular environment for Babies A and B was already significantly compromised before birth, and that the postnatal deteriorations were a foreseeable consequence of that antenatal picture rather than an anomaly requiring a non-natural explanation.
Supporters of a CCRC referral argue that the failure to put the full TTTS and monochorionic-twin literature before the jury, and the failure to cross-examine the prosecution’s experts on the antenatal Doppler trajectory, represents a material gap in the trial. The Panel’s findings on this point are among the submissions being developed for a referral application.