Fetal Assessment: Key Evaluations & Prenatal Diagnosis

What all needs to be assessed?

• Fetal gestational age
• Prenatal diagnosis: Neural tube defects, Aneuploidies
•  Fetal growth and maturity
•  Fetal well-being

Gestational Age Assessment

1. Clinical estimations: Based upon the last menstrual period (LMP).
   • Expected delivery date (EOD): Add 280 days to the first day of LMP.
2. Ultrasound
   • Most accurate for GA estimation.
   • First trimester USG is more accurate and superior to later-age USGs.
   • 1 trimester: Crown-rump length estimation is superior. When gestational age is calculated from this method, the accuracy is within 5-7 days of the actual gestational age.
   • 2 trimesters: BPD, HC, AC, and FL, if done later, are more accurate. When gestational age is calculated from this method, the accuracy is within 10–14 days of the actual gestational age.
   • 3 trimesters: BPD, HC, AC, FL. The accuracy of gestational age is within 21–30 days of actual gestational age.

Also read : WHO Recommendations For Care Of The Preterm Or Low Birth Rate Infants-2022 

Prenatal Diagnosis of Fetal Diseases

Investigations can be divided into two parts:

• Screening: Non-invasive Example: MSAFP (Maternal Serum Alpha-Fetoprotein)
• Diagnostic: Invasive Example: CVS (Chorionic Villus Sampling)

Screening for Open Neural Tube Defects (NTDs)

• USG: operator dependent.
• MSAFP (Maternal Serum Alpha Fetoprotein).

Time to check for neural tube defects: 15-22 weeks of gestation, Interpretation of MSAFP Significant elevation: elevated >2.5 MOM (Multiples of the Median) for gestational age—MSAFP is significantly elevated in 95% cases of anencephaly and elevated in 70-85% cases of open NTDs. (Neural tube defects). Closed NTDs like Spina bifida occulta-elevation elevationmight not be significant and can miss many of the nonopen NTDs

False elevation in MSAFP: It means there are no NTDs, but we're finding MSAFP to be >2.5 MOM (significantly elevated).

• The commonest reason for this is the wrong gestational age assessment.
• Suspecting false elevation in MSAFP-USG is done, followed by gestational age assessment and looking for the other signs of NTDs (lemon sign, banana sign).

Also read : Childhood Pneumonia- Causes, Symptoms, and Treatment 

Screening for Aneuploidies 

1st trimester, there are four ways:

1. Maternal age

Increased risk of aneuploidies (especially trisomy 21) is found to be associated with increased maternal age. It is not very accurate. It is not done as a standalone investigation.

2. Ultrasonography:

 particular marker checked is nuchal translucency/NT (at the back of the neck of the fetus, check for thickness of the nuchal fold.) presence of edema in the nuchal fold or nuchal translucency is a significant marker

Conditions with increased NT

- Aneuploidies (70–80% detection rate), Congenital Heart Diseases, Congenital Diaphragmatic Hernia, Fetal infections, and exophthalmos

β- HCG level:

•  Elevated in Trisomy- 21
•  Decreased in Trisomy- 18, 13

PAPP-A (Pregnancy Associated Plasma Protein-A):

Decreased in all the trisomies

According to NNF (National neonatology forum),

Measuring Nuchal Translucency

The ideal time to check for nuchal translucency is 11 weeks 0 days to 13 weeks 6 days.

• CRL (crown-rump length), when performed, should be 45-84 mm.
• Interpretation of NT becomes difficult, 2nd trimester onwards.

Other First Trimester Screening Procedures for Aneuploidies:

USG Markers: absent nasal bone (seen in about 60% cases of Trisomy-21, 50% cases of Trisomy-18, 40% cases of Trisomy-13).

• Absent nasal bone is seen in 1-2% cases in normal pregnancy.

Doppler showing reversal of A-wave flow in ductus venosus.

• Doppler showing tricuspid regurgitation at 11–13 weeks (55% positivity for Trisomy-21, 30% cases of Trisomy-18, 30% cases of Trisomy-13).

2nd Trimester Screeningo Triple Test

1. Combination of 3 tests
   • MSAFP
   • b-HCG level
   • Unconjugated Estriol (Ue ) 3
2. Used to check for Trisomy-21.
   • MSAFP levels are decreased
   • b-HCG levels are increased
   • Unconjugated estrogen (UE) levels are decreased.

Quadruple Test:

• Done between 15-22 weeks, but it is best done between 16-18 weeks.
• Triple Test + Inhibin-A levels (increased in Trisomy 21).
• More frequently done tests.

Integrated Test:

 1st trimester combined test (minus b-HCG) + second trimester quadruple test.

 Ideally, the b-HCG test is done only once.

 Also read : Staphylococcus Infections in Children: Signs and Treatment 

What Is Sequential Screening?

Sequential screening is of two types:

1. Stepwise:
   • First-trimester: Check for NT and PAPP-A levels.
   • Risk assessment.
   • Positive (high risk); negative (medium to low risk).
   • For high risk, → Do inan invasiveest.
   • Medium to low risk → All children/pregnancies will undergo the 2nd trimester quadruple test (noninvasive test).
   • 95% sensitivity in picking up aneuploidies.

2. Contingent:
   • 1st trimester: Check for NT and PAPP-A levels.
   • Risk assessment.
   • Positive (high risk)
   • Negative (medium to low risk).
   • High-risk → invasive test.
   • Medium-risk → Quadruple testing needs to be done.
   • Low risk→ No further testing needs to be done.
   • 93% sensitivity in picking up aneuploidies.

Non-Invasive Prenatal Testing (NIPT) 

1. It is also called cell-free fetal DNA estimation.
2. A non-invasive screening test (not diagnostic as per Cloherty, can be diagnostic as per Nelson)
3. Done via analysis of cell-free fetal DNA (cfDNA) in maternal blood, using next-generation DNA sequencing.
4. Performed in:
   • >10-week GA, especially in women >35-year
   • History of aneuploid fetus
   • A positive or intermediate-risk 1st trimester test
   • Carriers of balance translocation.
5. If positive/high risk, perform invasive testing for confirmation.
6. Very high detection rate for
   • Trisomy 21 (>99% sensitivity and specificity)
   • Trisomy 18 (sensitivity 97.4% and specificity 99.8%)
   • Trisomy 13 (sensitivity 91%, specificity 99.6%)
7. False-positive rate of 0.1-0.2%.
8. Can also reliably detect microdeletion syndromes (Williams, DiGeorge syndromes, etc.)
9. Can miss mosaics.

Also read : Fungal Infections In Neonates 

FAQ’S

Q. How often are fetal assessments required?

Ans. The frequency of assessments varies based on individual risk factors and gestation age

Q. What can affect fetal assessment results?

Ans. Diabetes, high BP, fetal position, & amniotic fluid are the factors  in maternal health conditions.

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