biology

Twin-to-twin transfusion syndrome

Twin-to-twin transfusion syndrome is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Twin-to-twin transfusion syndrome rather than just read about it. In short: Twin-to-twin transfusion syndrome (TTTS), also known as feto-fetal transfusion syndrome (FFTS), twin oligohydramnios-polyhydramnios sequence (TOPS) and stuck twin syndrome, is a complication of monochorionic multiple pregnancies (the most common form of identical twin pregnancy) in which there is disproportionate blood supply between the fetuses. This leads to unequal levels of amniotic fluid between each fetus and…

Key takeaways

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Reference excerpt

Twin-to-twin transfusion syndrome (TTTS), also known as feto-fetal transfusion syndrome (FFTS), twin oligohydramnios-polyhydramnios sequence (TOPS) and stuck twin syndrome, is a complication of monochorionic multiple pregnancies (the most common form of identical twin pregnancy) in which there is disproportionate blood supply between the fetuses. This leads to unequal levels of amniotic fluid between each fetus and usually leads to death of the undersupplied twin and, without treatment, usually death or a range of birth defects or disabilities for a surviving twin, such as underdeveloped, damaged or missing limbs, digits or organs (including the brain), especially cerebral palsy. The condition occurs when the vein–artery connections within the fetuses' shared placenta allow the blood flow between each fetus to become progressively imbalanced. It usually develops between week 16 and 25 of pregnancy, during peak placental growth. The cause of the developmental effects on a surviving fetus may include necrotic embolisms from a dead fetus, low blood volume due to pooling in the dead fetus or velamentous cord insertion (insertion of the umbilical cord into the chorioamniotic membranes). The primary treatment of TTTS is fetoscopy and laser ablation of the interconnecting blood vessels to cut off the exchange of blood between the fetuses. This treatment is associated with an 85% survival rate of at least one fetus. Other treatments include periodic removal of amniotic fluid (serial amniocentesis), which is associated with a 66% survival rate of at least one fetus. Without treatment, there is an almost 100% mortality rate of one or all fetuses. Even with treatment, the condition is associated with premature birth and a risk of cerebral palsy in a surviving fetus. Around 5–15% of identical twin fetuses will go on to develop TTTS. The condition was first described by German obstetrician Friedrich Schatz in 1875.

Cause As a result of sharing a single placenta, the blood supplies of monochorionic twin fetuses can become connected, so that they share blood circulation: although each fetus uses its own portion of the placenta, the connecting blood vessels within the placenta allow blood to pass from one twin to the other. It is thought that most monochorionic placentae have these "shared connections" that cross the placenta, with the net flow volumes being equal between them. This state is sometimes referred to as "flow balance". When the placenta has deep vein–artery connection, this can cause blood flow to become unbalanced. Depending on the number, type and direction of the interconnecting blood vessels (anastomoses), blood can be transferred disproportionately from one twin (the "donor") to the other (the "recipient"), due to a state of "flow imbalance" imparted by new blood vessel growth across the placental "equator", the line that divides each baby's proportion of the shared placenta. This state of transfusion causes the donor twin to have decreased blood volume, retarding the donor's development and growth, and also decreased urinary output, leading to a lower than normal level of amniotic fluid (becoming oligohydramnios). The blood volume of the recipient twin is increased, which can strain the fetus's heart and eventually lead to heart failure, and also higher than normal urinary output, which can lead to excess amniotic fluid (becoming polyhydramnios). The demise of the fetus is typically a result of ischemia related to the lack of blood flow. The lack of blood flow causes bowel atresia, brain damage, and kidney failure. TTTS usually develops during the period of peak placental growth, starting in week 16 and proceeding through week 25; after this point, the placenta's growth decelerates, essentially stopping just after week 30. While TTTS has occasionally been detected beyond this timepoint, it is thought that its occurrence beyond week 30 may be due to a placental embolism that upsets the flow balance of the shared connections between the babies. TTTS is potentially lethal to either or both twins, no matter when it is detected. However, when detected past week 25, emergency delivery may be considered to rescue the babies if the TTTS is severe. Other than requiring a monochorionic twin (or higher multiple) pregnancy, the underlying causes of TTTS are not known. It is not known to be hereditary or genetic.

Effects to the surviving fetus The fetal demise of one of the twins during the second trimester of a monochorionic pregnancy can result in serious complications to the surviving fetus. Complications include gangrenous limbs, hands and feet, cerebral palsy and IQ deficits, constriction rings of limbs and digits, reduced digits, skin defects, brain cysts, hydranencephaly, multicystic encephalomalacia, microencephaly, renal agenesis and bowel atresia. There are three hypotheses explaining these complications.

Embolic Theory: The demise of the donor twin allows necrotic tissue, presumably thrombosis-plastin like material, to pass through the vasculature of the surviving twin via the placental vessels. The necrotic tissue embolizes the surviving twin's mesenteric artery, resulting in bowel atresia, the renal artery, resulting in renal agenesis, or the peripheral arteries, resulting in limb ischemia and skin abnormalities. Ischemic Theory: The demise of the donor twin creates a low-pressure state resulting in the shunting of blood into the low-resistance vascular system of the dead fetus. This state of low pressure acutely causes hypovolemia, resulting in ischemia and poor end-organ perfusion. Placentation Theory: The incidence of velamentous cord insertion, an abnormal insertion of the umbilical cord into the placenta, is higher in TTTS. The exact mechanism of effect this has on fetal damage is unknown.

Diagnosis

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Illustrations

Worked examples

Example 1 — a first encounter with Twin-to-twin transfusion syndrome

Start with the simplest possible case. Write down what Twin-to-twin transfusion syndrome claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Twin-to-twin transfusion syndrome before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Twin-to-twin transfusion syndrome ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Twin-to-twin transfusion syndrome

In research: Twin-to-twin transfusion syndrome appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Twin-to-twin transfusion syndrome in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Twin-to-twin transfusion syndrome is common in secondary-school and first-year university syllabi. It links to neighbouring topics Pathology of pregnancy, childbirth and the puerperium, Syndromes, Transfusion medicine, so understanding it makes those chapters shorter.
In everyday life: Look for Twin-to-twin transfusion syndrome outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Twin-to-twin transfusion syndrome in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Twin-to-twin transfusion syndrome means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Twin-to-twin transfusion syndrome out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Twin-to-twin transfusion syndrome in simple terms?

Why does Twin-to-twin transfusion syndrome matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Twin-to-twin transfusion syndrome?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Twin-to-twin transfusion syndrome.