Monochorionic twin pregnancy

A 24-year-old woman at 18 weeks with a monochorionic diamniotic twin pregnancy arrives with sudden abdominal distension and dyspnoea. The scan shows a deepest vertical pool of 11 cm around the recipient and a stuck donor with no visible bladder. This is Quintero stage II twin-twin transfusion syndrome, and the next twenty-four hours decide whether this pregnancy is salvageable. The decisions — chorionicity, surveillance, staging, fetal therapy and the timing of birth — are the decisions this topic rehearses.^[2][3]

Overview and definition

A monochorionic twin pregnancy is one in which two fetuses share a single placenta. The shared placenta carries a network of vascular anastomoses that allow inter-twin blood transfer, and those anastomoses are the engine of every complication that makes this pregnancy different.^[1][3]

Three complications dominate and they share a vascular cause but present, stage and are managed differently:^[1][2]

• Twin-twin transfusion syndrome (TTTS) — unbalanced AV flow producing oligo-polyhydramnios in roughly 8 to 10 per cent of monochorionic pregnancies, usually between 16 and 26 weeks.
• Twin anemia-polycythemia sequence (TAPS) — chronic slow transfusion through tiny AV anastomoses, with hemoglobin discordance but normal amniotic fluid volumes, in roughly 3 to 5 per cent spontaneously and 3 to 16 per cent after selective laser.^[8][10]
• Selective fetal growth restriction (sFGR) — unequal placental sharing, in 10 to 15 per cent of monochorionic twins, with a disproportionate share of perinatal mortality.^[16][17]

The single most important action in any twin pregnancy is to assign chorionicity in the first trimester — because every downstream decision (surveillance frequency, threshold for referral, planned birth gestation) flows from it.^[2][15]

References20ShowHide

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14. [14]American College of Obstetricians and Gynecologists' Committee on Practice Bulletins-Obstetrics Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies: ACOG Practice Bulletin, Number 231 Obstet Gynecol, 2021.PMID 34011891
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