[CG] Twin pregnancy - ultrasound guideline


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  • Referral should be made for counselling for antenatal screening for combined trisomy (21,18,13) following the diagnosis of a twin pregnancy (fetal medicine at QEUH, Day Care counselling midwife PRM, Consultant RAH). This should ideally be prior to the 11+2 to 14+1 week scan.
  • An ultrasound examination should be performed between 11+2 and 14+1 weeks of gestation (CRL 45-84mm) to assess viability, gestational age and chorionicity +/- NT screening for trisomy.
  • The largest baby should be used to calculate the estimated date of delivery for the pregnancy.
  • Chorionicity assessment is based upon the number of placental masses, the appearance of the membrane attachment and membrane thickness, ideally before 14 weeks gestation.
  • A photographic record should be placed in the patient’s hospital held records documenting the ultrasound appearance of the membrane attachment to the placenta and an electronic / hard drive record stored.
  • If there is uncertainty about the chorionicity, a photographic record should be taken and of the ultrasound appearance of the attachment of the membrane to the placenta and a second opinion sought.
  • If there is still doubt about the chorionicity, the woman should be referred to medical staff for chorionicty assessment without delay.
  • Following this if there is still doubt, the pregnancy should be managed as monochorionic until proved otherwise.
  • The fetuses should be assigned nomenclature (ie upper and lower, right or left) and this should be clearly documented in the case notes to ensure consistency throughout pregnancy.
  • Women who ‘miss’ or have unsuccessful first trimester screening for aneuploidy should be offered second trimester screening.

Monochorionic Twins

Fetal ultrasound assessment should be performed every two weeks in uncomplicated monochorionic twins from 16+0 weeks onwards until delivery.

Scans at 16 and 20 weeks (detailed anomaly scan) should be performed by a medical sonographer. The detailed fetal anomaly scan should include extended cardiac views (5 standard views).

At every ultrasound, liquor volume (LV) should be assessed in each sac and deepest vertical pool (DVP) measured and recorded. Umbilical artery pulsatility index (UAPI) should be assessed and recorded. Fetal bladders should be visualised. Middle Cerebral Artery Peak Systolic Velocity (MCA PSV) should be recorded at each assessment.

  • From 16+0 weeks fetal biometry should be assessed and abdominal circumference (AC) recorded for each twin. If the difference in AC measurements is greater than 20mm, the AC discordance should be calculated- (larger twin AC-smaller twin AC]/larger twin AC) x100.
  • An AC discordance of more than 20% should prompt medical review. Onward referral to fetal medicine is indicated as these pregnancies have an increased perinatal risk. UAPI/MCA PSV and Ductus Venosus (DV) Dopplers should be performed. Pregnancies with absent or reverse EDF (AREDF) and ‘cyclical’ umbilical artery waveforms (intermittent AREDF) have an increased risk of perinatal morbidity and mortality.
  • Refer for medical scan if LV DVP>8 cm or <2cm before 20 weeks and LV DVP >10cm or <2cm after 20 weeks. If abnormality confirmed discussion with fetal medicine at QEUH is indicated.
  • If features of TTTS present, perform UAPI, MCA PSV and DV Dopplers and refer to medical staff that day with a view to assessment. Onward referral should be made to fetal medicine at QEUH.

Dichorionic Twins

Scan 20 weeks

  • Routine FAS

Scans thereafter every 4 weeks for growth unless clinical concern

  • Perform umbilical artery Dopplers (UAPI) from 24 weeks unless concerns prior to this.

Staging of Twin-to-twin transfusion syndrome (TTTS)

Stage Description
I Poly/oligohydramnios with bladder of the donor still visible
II Bladder of the donor no longer visible
III Presence of either absent or reverse end-diastolic velocity of the umbilical artery, reverse flow in either twin
IV Hydrops in either twin
V Demise of one or both twins prior to surgery

Selective intrauterine growth restriction (growth discordance of >20%). Approximately 10-15 % of MCDA twins

Stage Description
I Growth discordance but positive diastolic velocities in both fetal
umbilical arteries.
II Growth discordance with absent or reversed end-diastolic velocities (AREDV) in one or both fetuses.
III Growth discordance with cyclical umbilical artery diastolic waveforms (positive followed by absent then reversed end-diastolic flow in a cyclical pattern over several minutes [intermittent AREDV; iAREDV]).

Appendix: MONOCHORIONIC DIAMNIOTIC TWINS – antenatal appointments

Appendix: DICHORIONIC DIAMNIOTIC TWINS –antenatal appointments


Last reviewed: 10 August 2017

Next review: 01 August 2022

Author(s): Donna Maria Bean

Approved By: Obstetric Clinical Governance Group