Fifteen percent of spontaneous twin pregnancies and almost 5 % of the medically assisted twin pregnancies are monochorionic–diamniotic (MCDA) [1].

Fifteen to twenty percent of monochorionic–diamniotic twin pregnancies are complicated by twin-to-twin transfusion syndrome (TTTS) [1].

Twin-to-twin transfusion syndrome (TTTS) is a specific complication of monochorionic pregnancy resulting from an imbalance in the blood flow between the twins sharing the placenta, leading to unequal distribution of the blood flow such that one twin is compromised and the other is favored. Blood from one twin (the donor) is “transfused” into the other twin (the recipient) via connecting blood vessels within their common placenta. It has life-threatening effects upon both twins with perinatal mortality approaching 90 % if untreated [2–4].

Differential cord insertion of the MCDA twins with unequal sharing of the placental disk forms the basis of TTTS. Inter-twin vascular anastomoses as shown in Fig. 37.2 are present in virtually all monochorionic pregnancies. Arteriovenous anastomoses (labeled 1 in Fig. 37.2) with unidirectional flow, arterioarterial (labeled 2 in Fig. 37.2), or venovenous anastomoses with bidirectional flow are seen in the placenta of MCDA twins. Arteriovenous anastomoses are usually situated deeply in the placenta. Bidirectional anastomoses (arterioarterial, venovenous – both usually located on the placental surface) can assist in regulating unbalanced, unidirectional flow which occurs through the arteriovenous anastomoses and are protective against TTTS. Thrombosis of the superficial vessels may give rise to TTTS [2–4] (Fig. 37.3).

The asymmetric, bidirectional inter-twin exchange of blood and biochemical components that occurs between the donor and recipient twins results in severe hemodynamic, osmotic, and physiologic changes in both twins resulting in renal structural and functional aberrations in the donor twin and cardiovascular compromise in the recipient twin as depicted in the flowchart below [5].

Characteristic ultrasound findings enable prenatal diagnosis of TTTS. With advances in first trimester ultrasound and the widespread use of first-trimester screening, affected pregnancies are becoming increasingly diagnosed at earlier gestational ages. The first step in prenatal diagnosis is early and accurate determination of chorionicity as shown in ultrasound (USG) Image 37.1.

The diagnostic criteria on ultrasound are:

In the FIRST TRIMESTER

In the SECOND TRIMESTER

Though all monochorionic pregnancies are considered “high risk” and need intensive surveillance, Lewi et al. have recommended a method of risk stratification of monochorionic pregnancies based on ultrasound findings.

In the first trimester, if there is discordant amniotic fluid or a discordance in crown–rump length (CRL) of ≥12 mm, these pregnancies are categorized as “high risk.” Monochorionic pregnancies with concordant amniotic fluid and difference in CRL of <12 mm are labeled as “low risk” [7].

In the second trimester (after 16 weeks), a high risk of adverse outcome was predicted by the presence of discordant amniotic fluid and discordant cord insertions. Alternatively, for cases with only discordant fluid but concordant cord insertions, adverse outcome was predicted by an inter-twin difference in abdominal circumference of >6 mm and for cases with concordant fluid but discordant cords by a difference in abdominal circumference >13 mm. Finally, in the absence of both discordant fluid and cord insertions, an adverse outcome was predicted by a difference in abdominal circumference >24 mm [7].

The “high-risk” pregnancies have >70 % risk of adverse outcome and <70 % survival rate unless treated. They will need weekly surveillance in a tertiary care center and have to be offered invasive fetal therapy [7].

The “low-risk” pregnancies have a 90 % survival rate with <10 % incidence of complications. They are followed up fortnightly in a tertiary care center [7].

TTTS is a dynamic condition which may remain unchanged, progress slowly, or develop quickly with rapid deterioration in fetal well-being hence the importance of serial surveillance of MCDA twin pregnancies, preferably in a maternal fetal medicine unit.

The Quintero staging for TTTS was proposed in 1999 and is internationally recognized. It helps in early diagnosis and appropriate referral for treatment. The main limitations are in the prediction of progression and outcome. It indirectly considers cardiovascular changes in recipient twin via the ductus venosus Doppler. The staging is as follows:

The newer staging systems [Cincinnati Staging system, Cardiovascular Profile Scoring system (CVPS), Children’s Hospital of Philadelphia system (CHOP)] incorporate echocardiographic criteria in order to overcome the limitations of the Quintero staging system, but are more complex and not widely followed in clinical practice currently [9]. Quintero staging system considers the pregnancy as a whole and gives a pregnancy specific risk, whereas the newer classifications offer an individual risk to each fetus. It also minimizes inter- and intraobserver variation. It is important to remember that patients could change from one stage to another rapidly and will not necessarily follow the sequence as specified by Quintero.