Twin pregnancies are categorized by their origin and placental structure, with monochorionic types facing the risk of Twin-to-Twin Transfusion Syndrome (TTTS) due to shared placental circulation. TTTS arises from unbalanced blood flow between the twins, impacting donor and recipient fetuses differently. Diagnosis relies on ultrasound assessments of amniotic fluid and fetal bladder, and management is tailored to the severity, potentially involving laser surgery for advanced stages.

Twin pregnancies are categorized by their origin and placental structure, with monochorionic types facing the risk of Twin-to-Twin Transfusion Syndrome (TTTS) due to shared placental circulation. TTTS arises from unbalanced blood flow between the twins, impacting donor and recipient fetuses differently. Diagnosis relies on ultrasound assessments of amniotic fluid and fetal bladder, and management is tailored to the severity, potentially involving laser surgery for advanced stages.

Twin pregnancies are categorized by their origin and placental structure, with monochorionic types facing the risk of Twin-to-Twin Transfusion Syndrome (TTTS) due to shared placental circulation. TTTS arises from unbalanced blood flow between the twins, impacting donor and recipient fetuses differently. Diagnosis relies on ultrasound assessments of amniotic fluid and fetal bladder, and management is tailored to the severity, potentially involving laser surgery for advanced stages.

Twin births account for about 2 to 4 per cent of births worldwide. Twin pregnancies are broadly classified as either dizygotic, which arise from two separate ova, or monozygotic, which develop from a single fertilized ovum.

Based on the number of placentas and amniotic sacs, twin gestations are further categorised as dichorionic-diamniotic (DCDA) with two placentas and two amniotic sacs, monochorionic-diamniotic (MCDA) with one shared placenta and two amniotic sacs, and monochorionic-monoamniotic (MCMA) with one shared placenta and one amniotic sac. Approximately 67 per cent of twin pregnancies are dizygotic, while 33 per cent are monozygotic.

One of the most important concerns in monochorionic pregnancies is that more than 95 per cent of MCDA twins share placental circulation. This shared blood supply creates the possibility of developing Twin-to-Twin Transfusion Syndrome (TTTS), a condition that is unique to monochorionic twin pregnancies and requires close monitoring throughout pregnancy.

What causes Twin-to-Twin Transfusion Syndrome?

TTTS is a serious complication caused by unbalanced blood flow between twins sharing a single placenta. It occurs in approximately 8 to 12 per cent of MCDA twin pregnancies and about 6 per cent of MCMA twin pregnancies.

The underlying cause is the presence of an increased number of deep arteriovenous anastomoses within the placenta. These vascular connections allow blood to flow in one direction, resulting in blood being diverted from one fetus, known as the donor twin, to the other, known as the recipient twin. As a result, the donor twin may receive inadequate blood volume, while the recipient twin receives an excessive amount, placing stress on both fetuses.

Diagnosis and ultrasound surveillance

Early diagnosis of TTTS relies on careful ultrasound evaluation in monochorionic pregnancies. The diagnosis can be made when one gestational sac demonstrates oligohydramnios, defined as a maximum vertical pocket (MVP) of less than 2 cm, while the other sac shows polyhydramnios, with an MVP greater than 8 cm. Although fetal growth restriction or growth discordance may be present, these findings are not necessary for diagnosing TTTS.

Routine ultrasound surveillance should include assessment of the amniotic fluid volume in both sacs, evaluation of the presence or absence of a urine-filled bladder in each twin, and, ideally, Doppler studies of the umbilical arteries to assess fetal circulation. Regular surveillance allows timely recognition of disease progression and helps guide treatment decisions.

Staging and management of TTTS

TTTS is classified into five stages according to its severity.

Stage I is characterised by oligohydramnios in the donor twin and polyhydramnios in the recipient twin.

Stage II is diagnosed when the bladder of the donor twin is no longer visible on ultrasound.

Stage III involves abnormal Doppler findings in either twin, indicating worsening fetal circulation.

Stage IV is marked by the development of hydrops fetalis in either twin.

Stage V represents intrauterine demise of one or both fetuses.

Management depends on the stage of the condition. For asymptomatic Stage I TTTS, expectant management with at least weekly fetal surveillance is generally recommended. For Stage II through Stage IV, fetoscopic laser surgery is considered the standard treatment, as it seals the abnormal placental vascular connections responsible for the unequal blood flow. In Stage V TTTS, immediate delivery of the surviving twin has not been shown to improve outcomes. Unless there are additional obstetric indications, preterm delivery should be avoided.

Early diagnosis, regular ultrasound surveillance, and timely intervention remain the cornerstone of managing TTTS and improving outcomes for both twins in monochorionic pregnancies.

(The author is a consultant, fetal medicine & clinical genetics, Manipal Hospital, Bhubaneshwar)

The opinions expressed in this article are those of the author and do not purport to reflect the opinions or views of THE WEEK.

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