Antenatal screening and IPT

Question 1

Describe combined first trimester screening

Answer 1

• Performed between 11+0 and 13+6 weeks
• MSS-1: BhCG, PAPP-A
• Maternal age
• Nuchal translucency scan
• High sensitivity 85% esp T21

Question 2

Describe maternal serum screen-2

Answer 2

• Maternal blood test: oestradiol, inhibin A, AFP, BhCG
• Maternal age
• Performed between 15 to 20 weeks.
• Less sensitive than combined first trimester screening 70%

Question 3

Describe cell free fetal DNA testing (NIPT)

Answer 3

• Uses:
  
  • Primary screening
  • Secondary test after a high risk first trimester screening result.
• Maternal blood test that analyses fetal DNA in the maternal blood sample.
• Performed from 10 weeks.
• Highest sensitivity 99.9% for T21
• Also for: T13, T18, sex chromosome aneuploidy, cystic fibrosis, spinal muscular atrophy and fetal sex.
• 5% no result due to low fetal fraction.
• Self-funded
• Can get false positives/negatives due to confined placental mosaicism.

Question 4

Describe CVS

Answer 4

• Chorionic villus sampling: under ultrasound guidance a needle is passed through the abdomen and into the placenta where placental villi are sampled.
• Time: from 11 weeks.
• Tests:
  
  • Karyotype
  • Microarray
• Risks:
  
  • Miscarriage <1%
  • Bleeding
  • Rh sensitisation - needs Anti-D
  • False positive or negative from confined placental mosaicism 1-2%; needing amnio after.

Question 5

Describe amniocentesis

Answer 5

• Amniocentesis: a needle is passed under ultrasound guidance into the uterus and amniotic fluid is sampled.
• Time: 15 weeks
• Tests:
  
  • Karyotype
  • Microarray
  • Infection PCR
• Risks:
  
  • Miscarriage <1% and < than for CVS.
  • Bleeding
  • Infection
  • Amniotic fluid loss
  • Rh sensitisation - needs Anti-D