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]

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1. [1]Kilby MD, Bricker L; Royal College of Obstetricians and Gynaecologists Management of Monochorionic Twin Pregnancy Green-Top Guideline No. 51 (2024 Partial Update) BJOG, 2025.PMID 39966091
2. [2]Khalil A, Sotiriadis A, Baschat A, et al. ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy Ultrasound Obstet Gynecol, 2025.PMID 39815396
3. [3]Miller RS, Miller JL, Monson MA, et al. Society for Maternal-Fetal Medicine Consult Series #72: Twin-twin transfusion syndrome and twin anemia-polycythemia sequence Am J Obstet Gynecol, 2024.PMID 39029545
4. [4]Senat MV, Deprest J, Boulvain M, et al. Endoscopic laser surgery versus serial amnioreduction for severe twin-to-twin transfusion syndrome N Engl J Med, 2004.PMID 15238624
5. [5]Slaghekke F, Lopriore E, Lewi L, et al. Fetoscopic laser coagulation of the vascular equator versus selective coagulation for twin-to-twin transfusion syndrome: an open-label randomised controlled trial Lancet, 2014.PMID 24613024
6. [6]Slaghekke F, Lewi L, Middeldorp JM, et al. Residual anastomoses in twin-twin transfusion syndrome after laser: the Solomon randomized trial Am J Obstet Gynecol, 2014.PMID 24813598
7. [7]Kontopoulos E, Chmait RH, Quintero RA Twin-to-Twin Transfusion Syndrome: Definition, Staging, and Ultrasound Assessment Twin Res Hum Genet, 2016.PMID 27203605
8. [8]Lopriore E, Middeldorp JM, Oepkes D, et al. Twin anemia-polycythemia sequence in two monochorionic twin pairs without oligo-polyhydramnios sequence Placenta, 2007.PMID 16516289
9. [9]van den Wijngaard JP, Lewi L, Lopriore E, et al. Modeling severely discordant hematocrits and normal amniotic fluids after incomplete laser therapy in twin-to-twin transfusion syndrome Placenta, 2007.PMID 17098282
10. [10]Tollenaar LSA, Slaghekke F, Lewi L, et al. Spontaneous twin anemia polycythemia sequence: diagnosis, management, and outcome in an international cohort of 249 cases Am J Obstet Gynecol, 2021.PMID 32730900
11. [11]Barrett JF, Hannah ME, Hutton EK, et al. A randomized trial of planned cesarean or vaginal delivery for twin pregnancy N Engl J Med, 2013.PMID 24088091
12. [12]Hofmeyr GJ, Barrett JF, Crowther CA Planned caesarean section for women with a twin pregnancy Cochrane Database Syst Rev, 2015.PMID 26684389
13. [13]Aviram A, Lipworth H, Asztalos EV, et al. Delivery of monochorionic twins: lessons learned from the Twin Birth Study Am J Obstet Gynecol, 2020.PMID 32592694
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
15. [15]Khalil A, Rodgers M, Baschat A, et al. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy Ultrasound Obstet Gynecol, 2016.PMID 26577371
16. [16]Sorrenti S, Yaghi O, Prasad S, et al. Perinatal outcome of monochorionic twin pregnancy complicated by selective fetal growth restriction: ISUOG vs Delphi diagnostic criteria Ultrasound Obstet Gynecol, 2026.PMID 42126513
17. [17]El Emrani S, Groene SG, Verweij EJ, et al. Gestational age at birth and outcome in monochorionic twins with different types of selective fetal growth restriction Prenat Diagn, 2022.PMID 35808908
18. [18]Khalil A, Prasad S, Woolfall K, et al. FERN: is it possible to conduct a randomised controlled trial of intervention or expectant management for early-onset selective fetal growth restriction in monochorionic twins BMJ Open, 2024.PMID 39153765
19. [19]Kurtz AB, Wapner RJ, Mata J, et al. Twin pregnancies: accuracy of first-trimester abdominal US in predicting chorionicity and amnionicity Radiology, 1992.PMID 1438758
20. [20]Shamshirsaz AA, Chmait RH, Stirnemann J, et al. Solomon versus selective fetoscopic laser photocoagulation for twin-twin transfusion syndrome: A systematic review and meta-analysis Prenat Diagn, 2023.PMID 36184777