Screening for Down's syndrome in multiple pregnancy

• 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

Screening for chromosomal conditions

Twin pregnancy

• offer women with a twin pregnancy information on and screening for Down's syndrome, Edwards' syndrome and Patau's syndrome as outlined in the NHS fetal anomaly screening programme (FASP)

Triplet pregnancy

• before offering screening for Down's syndrome, Edwards' syndrome and Patau's syndrome, give women with a triplet pregnancy information about:
  • the greater likelihood of Down's syndrome, Edwards' syndrome and Patau's syndrome in triplet pregnancy
  • the different options for screening
  • the increased false positive rate of screening tests in triplet pregnancy
  • their greater likelihood of being offered invasive testing
  • their 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, Edwards' syndrome and Patau's syndrome in trichorionic triplet pregnancy should:
  • map the fetal positions
  • use nuchal translucency and maternal age to screen for Down's syndrome, Edwards' syndrome and Patau's syndrome when crown–rump length measures from 45.0 mm to 84.0 mm (at approximately 11+2 weeks to 14+1 weeks)
  • calculate the chance of Down's syndrome, Edwards' syndrome and Patau's syndrome for each fetus
• refer women with a dichorionic and monochorionic triplet pregnancy who want to have screening for Down's syndrome, Edwards' syndrome and Patau's syndrome to a tertiary level fetal medicine centre
• do not use second trimester serum screening for Down's syndrome in triplet pregnancies

Reference:

• NICE (April 2024). Twin and triplet pregnancy