Introduction

Did you know that the health of a twin can be directly life-threatening to the health of the other twin in a monochorionic pregnancy (which is when the twins share the same placenta in the womb)? Twin-to-Twin Transfusion Syndrome (TTTS) is a health condition that highlights the link between two lives that share the same placenta.

The goal of this article is to shed light on the clinical presentations of TTTS, with a focus on the importance of its early diagnosis. This article also helps you know the early red flag signs so that you can get early help.

What is twin-to-twin transfusion syndrome (TTTS)?

TTTS is a serious health complication that can occur in twin pregnancies where both babies share one placenta (monochorionic twins). It is the result of an imbalanced blood flow between the foetuses.¹

If left untreated, TTTS is associated with very high mortality rates during pregnancy, due to foetal and/or obstetric complications. TTTS has been reported to have a prevalence of 10–15% (approximately 1 in 2000) in all monochorionic twins.²

Cause and effect of TTTS 

Monochorionic twins share a single placenta. In TTTS, an abnormal blood vessel connection is formed between the foetuses, leading to unbalanced blood flow between the two, meaning the donor twin (the smaller twin, who “gives” their blood supply to the larger twin) and the recipient twin (the larger and receiving twin) because of abnormal blood vessel formation in the placenta.³

The imbalance in blood flow causes progressive loss of blood volume in the donor twin, and blood volume overload and heart strain in the recipient twin.¹

How does TTTS present?

It presents differently in the donor twin and the recipient twin. However, both twins are at risk of complications. Brain damage may occur from the quick blood volume changes in both twins as well.¹

Clinical presentation of the donor twin

• Foetal growth restriction (FGR): Due to inadequate nutrient and oxygen supply to the donor twin because they aren’t receiving as much blood as they need, they may grow more slowly than expected. They can also have kidney problems called renal tubular dysgenesis¹
• Reduced amniotic fluid volume: At an early stage, the donor twin presents with a reduced amount of protective amniotic fluid surrounding it - this is called oligohydramnios. Severely reduced amniotic fluid levels may result in the “stuck twin” phenomenon, which is when the foetus’ movement and growth are impaired and the donor twin appears tightly wrapped in their amniotic sac¹
• Pallor and anaemia: In the most severe cases, the smaller twin (donor twin)  develops anaemia (low haemoglobin, which is what transports oxygen in your blood) and low blood oxygen levels¹
• Low blood pressure: To compensate, the foetus increases production of an enzyme called renin that lowers blood pressure¹

Clinical presentation of the recipient twin

• Increased amniotic fluid: Polyhydramnios (excessive amniotic fluid) may result in complications such as maternal discomfort and preterm labour¹
• Macrosomia: Overgrowth of the foetus due to excessive blood and nutrient supply
• Cardiovascular Complications: The recipient twin may develop increasing blood pressure and blood volume, which may lead to heart failure and the development of an often fatal condition called hydrops fetalis. Cardiomegaly (heart enlargement) also occurs due to the blood volume overload¹
• Plethora: Reddish discolouration of the skin due to increased red blood cell production¹

Diagnosis of TTTS

Assessment of viability, chorionicity (whether there are two placentas or one), crown-rump length, and nuchal translucency should be done using an ultrasound scan at 10-13 weeks of gestation for all women having a twin pregnancy.⁴

The use of foetal echocardiography (ECG) for screening of congenital heart disease in all monochorionic twins due to the increased risk of heart problems is important.⁴

• Ultrasound findings
  • Oligohydramnios in the donor twin and polyhydramnios in the recipient twin^{3, 4}
  • Size discrepancies and evidence of “stuck twin” in the donor twin¹
• Doppler studies
  • This technique assesses the pattern of blood flow in the umbilical artery and veins^{6, 12}
• Echocardiography
  • This helps the physician evaluate heart function, particularly in the recipient twin⁴

Stages of TTTS? 

The stages are classified based on severity using the Quintero Staging System, which is based on 2-dimensional ultrasound and Doppler study findings.

It is made up of 5 stages:¹²

UA-Umbilical Arteries, UV- Umbilical Vein, DV - Donor Vein

More than three-fourths of cases of Stage 1 TTTS remain stable or even regress without invasive interventions, with about 86% perinatal survival. Therefore, in early stages, the condition is sometimes treated with a watch-and-wait strategy.⁴

Differential diagnoses

• Selective Intrauterine Growth Restriction (sIUGR)

Selective intrauterine growth restriction (sIUGR) is when one twin has an estimated weight 10% lower than it should be at its age.¹² It is a common complication associated with monochorionic (MC) pregnancy and has a risk of intrauterine foetal death(IUFD) or adverse outcomes, including disability for both twins.¹

It occurs in about 10–15% of twin pregnancies. sIUGR results in severe growth restriction in the twin with the smaller share of blood flow placenta. sIUGR is classified based on the umbilical artery (UA) Doppler flow in the smaller twin.¹¹

• Twin Anaemia-Polycythemia Sequence (TAPS): 

Twin anaemia-polycythemia sequence (TAPS) occurs from a chronic imbalance in net blood transfusion towards one twin and away from the other, which causes large inter-twin haemoglobin differences resulting in low haemoglobin in one and high haemoglobin in the other.

Unlike in TTTS, there is no imbalance in amniotic fluid between the twins.⁹  It is diagnosed later in pregnancy compared to TTTS, and with a Doppler ultrasound.⁸ TTTS and TAPS are separate entities, but can be present simultaneously.

• Other differential diagnoses include:

Preterm premature rupture of membranes (PPROM) and premature rupture of membranes, and amniotic fluid discrepancy caused by an anomaly in one of the twins. Therefore, it is important to answer questions to rule out these conditions before making a diagnosis of TTTS.

Management and implications

Frequent monitoring of high-risk monochorionic pregnancies is the key to achieving favourable outcomes. Management options available for TTTS include:⁴ 

• With patient/watchful /supportive care, doctors can help manage heart failure and amniotic fluid imbalances
• Reduction of excess amniotic fluid in the recipient's sac (amnioreduction)
• Intentional septostomy (cutting) of the membrane between the two babies to equalise the amount of amniotic fluid for each
• Laser ablation of the placenta
• Selective reduction of the number of foetuses

These are understandably very difficult decisions to make, and counselling is available if you need it, and your medical team is there to help you make the best decision for you.

Summary

Twin-to-Twin Transfusion Syndrome (TTTS) is a difficult condition found in monochorionic twin pregnancies, i.e. one placenta for twins in the womb. TTTS starts with an imbalance in blood flow between the two twins, when there is an abnormal blood connection in the placenta.

Detecting clear signs and symptoms early in both the donor and recipient twins, with the help of advanced testing tools, is the key to better outcomes before and after birth.

Morbidity and mortality can be significantly reduced through full prenatal checks and clear plans for laser surgeries, such as fetoscopic laser surgery or amnioreduction (removal of excess amniotic fluid).