When should pregnant women be checked for trisomy via amniocentesis and Rh negative testing?

Prenatal Screening for Trisomy and Rh Status

Pregnant women should be offered screening for trisomies at two key timepoints: first trimester combined screening (11-13+6 weeks) and second trimester quadruple screening (15-20 weeks), with diagnostic testing via amniocentesis after 15 weeks for screen-positive results. 1, 2

Trisomy Screening Timeline and Options

First Trimester Screening (11-13+6 weeks)

• Combined first trimester screening includes:
  • Nuchal translucency (NT) measurement via ultrasound
  • Serum markers: PAPP-A and free beta-hCG or total/intact hCG
  • Detection rate: 75-80% for Down syndrome with 5% false-positive rate 1
  • Can detect 75-80% of Trisomy 18 fetuses with 1% false-positive rate 3

Second Trimester Screening (15-20 weeks)

• Multiple marker screening (quad screen) includes:
  • Maternal serum alpha-fetoprotein (MSAFP)
  • Human chorionic gonadotropin (hCG)
  • Unconjugated estriol (uE3)
  • Inhibin-A
• Optimal timing: 16-18 weeks gestation 2
• Detection rate: approximately 75% of Down syndrome cases in women younger than 35 years with 5% positive screening rate 2
• Detection rate: over 80% for women 35 and older 2

Cell-free DNA Testing (≥10 weeks)

• Can be performed anytime after 10 weeks gestation
• Superior detection rate (>99% for trisomy 21) compared to traditional screening 4
• May be used as primary screening or follow-up to abnormal first/second trimester screening 1, 4

Diagnostic Testing for Trisomies

Chorionic Villus Sampling (CVS)

• Timing: 10-13+6 weeks gestation 2
• Some physicians perform up to 13+6 weeks 2
• Provides definitive chromosome analysis

Amniocentesis

• Timing: After 15 weeks gestation 2, 5
• Gold standard for definitive diagnosis of chromosomal abnormalities 6
• Early amniocentesis (10-14 weeks) has been studied but has higher risks 5, 7
• Allows direct karyotyping of fetal cells

Management Algorithm for Trisomy Screening

1. First trimester (11-13+6 weeks):

   • Offer combined screening (NT + serum markers)
   • If screen positive: offer genetic counseling and option of CVS or wait for second trimester amniocentesis

2. Second trimester (15-20 weeks):

   • Offer quad screening if no prior screening or as part of integrated screening
   • If screen positive: offer genetic counseling and amniocentesis

3. Screen positive results:

   • Prompt communication to referring physician within one working day 2
   • Genetic counseling should be offered 2
   • Amniocentesis should be offered after 15 weeks 2

Rh Status Testing

• All pregnant women should have blood type and Rh status determined at first prenatal visit
• For Rh-negative women:
  • Administer Rh immune globulin at 28 weeks gestation (1500 IU; 300 mcg) 8
  • Administer another dose within 72 hours after delivery if infant is Rh-positive 8
  • Additional doses required after potentially sensitizing events:
    • Amniocentesis
    • Abdominal trauma
    • Miscarriage/abortion
    • Ectopic pregnancy

Special Considerations and Pitfalls

• Gestational age accuracy is critical for proper interpretation of screening results

  • If gestational age changes by 2+ weeks after ultrasound, results must be reinterpreted 2
  • If sample drawn before 15 weeks for second trimester screening, a new sample should be sent 2

• Maternal factors affecting screening:

  • Weight
  • Race
  • Insulin-dependent diabetes
  • Multiple gestation pregnancies 2, 1

• Soft markers on ultrasound:

  • Thickened nuchal fold (≥6mm between 15-20 weeks) is highly associated with trisomy 21 2
  • If isolated thickened nuchal fold is found with negative screening, consider cfDNA testing or amniocentesis 2

• Rh immune globulin administration pitfalls:

  • Never administer intravenously (intramuscular only)
  • Never administer to the neonate
  • Gluteal region should not be used as injection site due to risk of sciatic nerve injury 8
  • For large fetomaternal hemorrhage (>15 mL red cells), additional doses may be required 8

By following this structured approach to prenatal screening and testing for trisomies and Rh status, clinicians can provide appropriate care to optimize maternal and fetal outcomes.