screening for Down's syndrome in multiple pregnancy

Last reviewed 01/2018

• before screening for Down's syndrome offer women with twin and triplet pregnancies information about:
  • the greater likelihood of Down's syndrome in twin and triplet pregnancies
  • the different options for screening
  • the false positive rate of screening tests, which is higher in twin and triplet pregnancies
  • the likelihood of being offered invasive testing, which is higher in twin and triplet pregnancies
  • the greater likelihood of complications of invasive testing
  • the physical risks and psychological implications in the short and long term relating to selective fetal reduction
    
    
• healthcare professionals who screen for Down's syndrome in twin pregnancies should:
  • map the fetal positions
    
    
  • use the combined screening test (nuchal translucency, beta human chorionic gonadotrophin, pregnancy-associated plasma protein-A) for Down's syndrome when crown-rump length measures from 45 mm to 84 mm (at approximately 11 weeks 0 days to 13 weeks 6 days)
    
    
  • calculate the risk of Down's syndrome per pregnancy in monochorionic twin pregnancies
    
    
  • calculate the risk of Down's syndrome for each baby in dichorionic twin pregnancies
    
    
• healthcare professionals who screen for Down's syndrome in triplet pregnancies should:
  • map the fetal positions
    
    
  • use nuchal translucency and maternal age to screen for Down's syndrome when crown-rump length measures from 45 mm to 84 mm (at approximately 11 weeks 0 days to 13 weeks 6 days)
    
    
  • calculate the risk of Down's syndrome per pregnancy in monochorionic triplet pregnancies
    
    
  • calculate the risk of Down's syndrome for each baby in dichorionic and trichorionic triplet pregnancies

Notes:

• where first trimester screening for Down's syndrome cannot be offered to a woman with a twin pregnancy (for example, if the woman books too late in pregnancy) consider second trimester serum screening and explain to the woman the potential problems of such screening. These include the increased likelihood of pregnancy loss associated with double invasive testing because the risk of Down's syndrome cannot be calculated separately for each baby
• second trimester serum screening for Down's syndrome should not be used in triplet pregnancies
• offer women with twin and triplet pregnancies who have a high risk of Down's syndrome (use a threshold of 1:150 as defined by the NHS Fetal Anomaly Screening Programme [FASP] ) referral to a fetal medicine specialist in a tertiary level fetal medicine centre

Reference:

• 1) NICE (September 2011). Multiple pregnancy The management of twin and triplet pregnancies in the antenatal period