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Twin Pregnancy Ultrasound Guideline (330)

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Gestational Age, Chorionicity and Amnionicity

Women should be offered a first trimester USS (< 14+0 weeks) to estimate gestational age and determine chorionicity and amnionicity.

Chorionicity and amnionicity should be determined by the number of placental masses, the presence of amniotic membranes and membrane thickness, the lambda or T-sign.

Clear nomenclature should be assigned e.g inferior and superior, or left and right, in a twin or triplet pregnancy to ensure consistency throughout pregnancy.

If a multiple pregnancy presents after 14+0 weeks then determination of chorionicity and amnionicity could also take into account discordant fetal sex if required.

If TAUSS views are poor because of a retroverted uterus or a high BMI, use a transvaginal ultrasound scan to determine chorionicity and amnionicity.

The largest baby should be used to calculate the estimated date of delivery for the pregnancy.

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. Chorionicity must be checked by senior sonographer

If there is still doubt about the chorionicity, the woman should be referred to medical staff for chorionicity assessment without delay.

Following this if there is still doubt, the pregnancy should be managed as monochorionic until proved otherwise.

Screening

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, Screening midwife Clyde). This should ideally be prior to the 11+2 to 14+1 week scan.

The test of choice for twin pregnancies is first trimester combined screening. Every opportunity must be made to maximise the offer of first trimester combined screening. Chance results to be reported are:

  • a term chance of T21 and a term chance of T18/T13
  • a term chance of T21 only
  • a term chance of T18/T13 only

First trimester combined screening will be reported in a dichorionic twin pregnancy as a chance for each fetus whereas in a monochorionic twin pregnancy it will be reported as a chance per pregnancy.

Women who ‘miss’ or have unsuccessful first trimester screening for aneuploidy should be offered second trimester screening for T21. Chance results are reported as a pregnancy related chance that is not fetal specific.

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, the following should be assessed and recorded:

  • liquor volume (LV) should be assessed in each sac and deepest vertical pool (DVP)
  • Umbilical artery pulsatility index (UAPI)*
  • Fetal bladders should be assessed.
  • Middle Cerebral Artery Peak Systolic Velocity (MCA PSV)

*See Umbilical Artery Pulsatility Index Chart

Increase the frequency of diagnostic monitoring for TTTS in the woman’s 2nd and 3rd trimester to at least weekly if there are concerns about differences between the babies’ amniotic fluid level (a difference in DVP depth of 4cm or more). Include Doppler assessment of the umbilical artery flow for each baby.

Refer for medical scan if LV DVP>8 cm or <2cm before 20 weeks or LV DVP >10cm or <2cm after 20 weeks. If abnormality confirmed discussion with fetal medicine at QEUH is indicated.

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

Stage Description

I

II

III

IV

V

Poly/oligohydramnios with bladder of the donor still visible

Bladder of the donor no longer visible

Presence of either absent or reverse end-diastolic velocity of the umbilical artery, reverse flow in either twin

Hydrops in either twin

Demise of one or both twins prior to surgery

From 16+0 weeks fetal biometry (HC, AC and FL) should be assessed and abdominal circumference (AC) and Estimated fetal weight (EFW) recorded for each twin. The discordance in EFW should be calculated and documented in monochorionic twins at each visit:

([EFW larger fetus − EFW smaller fetus] ÷ EFW larger fetus) × 100

Increase diagnostic monitoring in the 2nd and 3rd trimesters to at least weekly, and include Doppler assessment of the umbilical artery flow for each baby, if there is an EFW discordance of 20% or more and/or the EFW of any of the babies is below the 10th centile for gestational age.

Refer women with a monochorionic twin pregnancy to a tertiary level fetal medicine centre if there is an EFW discordance of 25% or more and the EFW of either of the babies is below the 10th centile for gestational age because this is a clinically important indicator of selective fetal growth restriction.

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

Stage Description

I

II

III

Growth discordance but positive diastolic velocities in both fetal umbilical arteries.

Growth discordance with absent or reversed end-diastolic velocities (AREDV) in one or both fetuses.

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]).

Offer weekly USS monitoring for TAPS from 16 weeks of pregnancy using middle cerebral artery peak systolic velocity (MCA-PSV) to women who pregnancies are complicated by:

  • feto-fetal transfusion syndrome that has been treated by fetoscopic laser therapy or
  • selective fetal growth restriction (defined by an EFW discordance of 25% or more and an EFW of any of the babies below the 10th centile for gestational age)

Aim for delivery between 36+0 and 36+6 for uncomplicated MCDA twins after which point continuing the pregnancy increases the risk of fetal death

For monochorionic monoamniotic twins birth should be planned between 32+0 and 33+6

Dichorionic Twins

Fetal anomaly scan can be performed by sonographer if there are no other obstetric reasons for a medical FAS.

Growth USS should be performed every 4 weeks from 24 weeks onwards. Estimated fetal weight, umbilical artery PI and deepest vertical pool of liquor should be measured at each visit.

Fetal weight discordance should be calculated for dichorionic twins:

([EFW larger fetus − EFW smaller fetus] ÷ EFW larger fetus) × 100

Increase monitoring in 2nd and 3rd trimesters to at least weekly, and include Doppler assessment of the umbilical artery flow for each baby if there is EFW discordance of 20% or more and/or the EFW of any of the babies is below the 10th centile for gestational age.

Refer women with a dichorionic twin pregnancy to a tertiary level fetal medicine centre if there is an EFW discordance of 25% or more and the EFW of any of the babies is below the 10th centile for gestational age because this is a clinically important indicator of selective fetal growth restriction.

Aim for delivery between 37+0 and 37+6 for uncomplicated DCDA twins as after this point continuing the pregnancy increases the risk of fetal death.

Appendix: Umbilical Artery Pulsatility Index (UAPI) Reference Chart

> 95th percentile is abnormal

Appendix: MONOCHORIONIC DIAMNIOTIC TWINS – antenatal appointments

(File in notes)

Monochorionic diamniotic twins - antenatal appointments - pdf

Appendix: DICHORIONIC DIAMNIOTIC TWINS –antenatal appointments

(File in notes)

Dichorionic diamniotic twins - antenatal appointments - pdf

Editorial Information

Last reviewed: 24/10/2023

Next review date: 31/10/2028

Author(s): Victoria Watson ST7 PRM, Donna Bean, Lead Sonographer Obstetrics and Gynaecology.

Version: 3

Approved By: Maternity Clinical Governance Group

Document Id: 330

References
  1. NICE [NG137. Twin and triplet pregnancy. 2019.
  2. Khalil A. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. Ultrasound in Obstetrics and Gynaecology. ISUOG. November 2015.
  3. NHS Fetal Anomaly Screening Programme (FASP). Public Health England. September 2019.