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Twin Pregnancy: Ultrasound Evaluation and Monitoring


Twin pregnancies are followed more closely than singleton pregnancies due to higher risk for complications such as twin-twin transfusion syndrome, selective fetal growth restriction, and preterm labor. Ultrasound is a non-invasive and highly useful tool for screening, diagnosis, and guiding management of these potential complications. Ultrasound monitoring protocols vary between different types of twin pregnancies.

Zygosity, Chorionicity and Amniosity

  • Zygosity: Describes genetic origin of twins
    • Dizygotic twins: Both twins originate from a separate oocyte and each oocyte is fertilized by its own spermatocyte
      • Will always be dichorionic and diamniotic
    • Monozygotic twins: Both twins develop from a single zygote which then cleaves to form two embryos
  • The timing of cleavage will determine chorionicity and amniosity of monozygotic twins
    • Approximately 2/3 monochorionic/diamniotic | 1/3 dichorionic/diamniotic | 1to 2% monochorionic/monoamniotic  
  • Chorionicity determines follow-up based on risks for adverse events

Image by Kevin Dufendach, MD (2008). Used by permission. CC BY 3.0

Dating (Ultrasound)

  • Ideal timing: Between 11w0d and 13w6d (45 and 84 mm)
    • Use CRL of the larger twin in spontaneously conceived twins
    • Use oocyte retrieval date or embryonic age from fertilization for twins conceived via IVF
  • Note: If a woman presents beyond 14 weeks gestational age then use head circumference of the larger twin (ISUOG)

Chorionicity Determination (Ultrasound)

  • Optimal timing to determine chorionicity by ultrasound is in the first or 2nd trimester
    • ACOG/SMFM recommend that chorionicity should be identified as early as possible
    • Ultrasound <13w6d may identify chorionicity in approximately 95% of cases based on membrane thickness and insertion
  • Dichorionic
    • Lambda (aka delta or twin peak) sign: Indicates dichorionic twins with thickening at membrane insertion site
    • 2 distinct placental masses
    • Discordant sex signifies dichorionic, diamniotic (and dizygotic) twins
  • Monochorionic
    • T sign: Indicates monochorionic twins, with thin membrane and absence of thickening at membrane insertion site
  • Note: A single placental mass does not rule out dichorionic twins

Lambda or Delta Sign Indicating Dichorionic Twins

© Nevit Dilmen Creative Commons Attribution-Share Alike 3.0 license

T Sign Indicating Monochorionic Twins

© Nevit Dilmen Creative Commons Attribution-Share Alike 3.0 license


Uncomplicated monochorionic twin pregnancy (see SMFM checklist in ‘Learn More – Primary Sources’ below)

  • 10 to 13 weeks:
    • NT | Size concordance
  • 16 weeks
    • AFV and bladder filling q2 weeks
    • Fetal growth q2 to 4 weeks
  • 18 to 22 weeks (earlier if possible)
    • Detailed anatomy and fetal echocardiogram
  • Antepartum surveillance (NST and/or BPP)
    • Monochorionic dichorionic: If fetal growth, AFV and bladder filling are all normal “there is no specific national recommendation regarding the need for, type of, or timing of surveillance”
    • Monochorionic monoamniotic: Initiation of surveillance typically at 32w0d | Clinicians may offer inpatient care starting at 24 to 28 weeks with daily surveillance but “optimal management remains uncertain” (ACOG)

Note: ISUOG guidelines do include umbilical artery Doppler monitoring as part of routine surveillance | ACOG/ SMFM considers evidence to be unclear for uncomplicate monochorionic twins

Uncomplicated dichorionic twin pregnancy

  • First trimester: Dating and chorionicity (see above)
  • 18 to 22 weeks: Detailed anatomy | Biometry | Amniotic fluid volume (AFV) | Cervical length
  • >20 weeks: q4 weeks until delivery: Fetal growth | AFV
  • 36w0d: consider weekly antenatal fetal surveillance


Most twin pregnancies will have good outcomes. However, diligence is required, especially in the case of monochorionic twins due to risk for twin-twin transfusion syndrome (TTTS) and twin anemia polycythemia sequence (TAPS). Monochorionic twins may have potentially significant vascular anastomoses such that the twins share a common vasculature. Significant risks for dichorionic twins include preterm labor, medical complications due to increased placental mass (e.g., preeclampsia and GDM) and selective growth restriction. Different centers will have different protocols for labeling twin A vs twin B. The important point is to be consistent with labeling.



  • 10 to 15% of monochorionic twins | 90% fetal demise if untreated
  • Diagnosis; AFV fluid imbalance noted on ultrasound
    • ‘Donor’ has oligohydramnios (DVP of < 2 cm) | ‘Recipient’ has polyhydramnios (DVP > 8 cm)


  • Can occur spontaneously in approximately 5% of monochorionic diamniotic twins
  • Result of small AV anastomoses that leads to transfusion of blood from donor to recipient twin
    • At birth, anemia in donor and polycythemia in the recipient
  • Prenatal diagnosis based on MCA Doppler
    • Donor MCA‐PSV > 1.5 MoM | Recipient MCA‐PSV < 1.0 MoM

When to Deliver (ACOG/SMFM)

  • Multiple gestation – uncomplicated
    • Di-di twins: 38w0d – 38w6d
    • Mono-di twins: 34w0d – 37w6d
    • Mono-mono twins: 32w0d – 34w0d
    • Note: Triplets and higher: Individualize
  • Multiple gestation – complicated by isolated FGR
    • Di-di twins: 36w0d-37w6d
    • Mono-di twins: 32w0d-34w6d
    • Note: If concurrent condition: Individualize

When to Refer

  • Monochorionic/ Monoamniotic twins
  • Growth discordance: Defined as EFW discordance is ≥ 20% (ACOG)
    • Calculation: Difference in the estimated fetal weight between the two fetuses/ divided by the weight of the larger fetus
    • ISUOG uses a 25% cut-off
  • Any findings on ultrasound that are of concern

Primary Sources – Learn More:

ACOG/ SMFM Practice Bulletin 231: Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies

SMFM Special Statement: Updated checklists for management of monochorionic twin pregnancy

ISUOG Practice Guidelines: role of ultrasound in twin pregnancy

ACOG SMFM Committee Opinion 831: Medically Indicated Late-Preterm and Early-Term Deliveries

Locate a Maternal Fetal Medicine Specialist

Maternal Fetal Medicine Specialist Locator-SMFM