What methods are recommended for diagnosing pregnancy and assessing fetal viability?

Pregnancy Diagnosis, Location, and Viability Assessment

Transvaginal ultrasound is the gold standard for diagnosing pregnancy location and viability, with optimal timing at 49 days (7 weeks) of gestation for conclusive assessment in asymptomatic women. 1

Diagnosis of Pregnancy

Initial Assessment

• Serum or urine β-hCG testing for initial pregnancy confirmation
• Transvaginal ultrasound (TVS) is the preferred imaging modality for early pregnancy assessment 2
• Timing of first ultrasound is critical for accurate diagnosis:
  • Before 35 days: Most commonly shows pregnancy of unknown location
  • 35-41 days: Early intrauterine pregnancy of uncertain viability
  • After 42 days: Viable intrauterine pregnancy can typically be confirmed 1

Optimal Timing for Ultrasound Assessment

• For asymptomatic women with no history of ectopic pregnancy, TVS should be delayed until 49 days (7 weeks) gestation to reduce inconclusive scans 1
• For women with symptoms (pain, bleeding) or history of ectopic pregnancy, earlier scanning is warranted as 72% of ectopic pregnancies present before 49 days 1

Confirming Pregnancy Location

Intrauterine Pregnancy Criteria

• Visualization of gestational sac within the uterine cavity
• Presence of yolk sac within the gestational sac confirms intrauterine location
• Double decidual sign (two concentric echogenic rings surrounding the gestational sac)

Ectopic Pregnancy Diagnosis

• Defined as pregnancy outside the uterine cavity
• Confirmed by ultrasound, surgical visualization, or histopathology 2
• Requires immediate intervention when diagnosed

Assessing Pregnancy Viability

Ultrasound Criteria for Viable Pregnancy

• A viable intrauterine pregnancy is defined as "a pregnancy confirmed by ultrasound examination of at least one fetus with a discernible heartbeat" 2
• Fetal cardiac activity is the definitive sign of viability
• Handheld transabdominal ultrasound can confirm viability from gestational week 7 with 100% positive predictive value, though sensitivity is lower than TVS 3

Ultrasound Criteria for Non-Viable Pregnancy

• Absence of cardiac activity when:
  • Crown-rump length (CRL) > 5 mm 4
  • Mean sac diameter (MSD) ≥ 16 mm 4
• Combination of absence of cardiac activity AND absence of yolk sac is highly specific for non-viability when MSD ≥ 16 mm 4
• Miscarriage can only be diagnosed on initial TVS after 35 days gestation 1

Advanced Assessment of Fetal Well-Being

Biophysical Profile (BPP)

For pregnancies beyond first trimester requiring detailed assessment:

• Consists of four ultrasound-based assessments: 2
  1. Fetal breathing movements (≥30 seconds within 30 minutes)
  2. Discrete body movements (≥3 movements)
  3. Fetal tone (≥1 episode of active extension with return to flexion)
  4. Amniotic fluid volume (≥1 pocket of 2 cm)
• Each component meeting criteria receives a score of 2, for a composite score out of 8
• Often combined with non-stress test (NST) for a total score out of 10
• Score interpretation: 8-10 normal, 6 equivocal, ≤4 abnormal 2

Modified BPP

• Combines amniotic fluid assessment with NST
• More streamlined approach than full BPP 2

Doppler Velocimetry

• Most valuable in assessing intrauterine growth restriction (IUGR)
• Umbilical artery Doppler findings predict perinatal outcomes in IUGR 2
• Abnormal findings include:
  • Decreased end-diastolic velocity
  • Absent end-diastolic velocity
  • Reversed end-diastolic velocity (most concerning)

Special Considerations

Fetal Growth Restriction Assessment

• Defined as ultrasonographic estimated fetal weight or abdominal circumference below the 10th percentile for gestational age 2
• Population-based fetal growth references (such as Hadlock) recommended for determining fetal weight percentiles 2
• Serial umbilical artery Doppler assessment recommended once IUGR is diagnosed 2
• Monitoring frequency based on severity:
  • Weekly for decreased end-diastolic velocity or severe IUGR (EFW <3rd percentile)
  • 2-3 times weekly for absent end-diastolic velocity
  • Daily monitoring with hospitalization for reversed end-diastolic velocity 2

Viability Limits

• In the United States, viability occurs at approximately 24 weeks of gestational age 5
• Survival rates:
  • 23 weeks: ~10% survival rate with high risk of disability
  • 24 weeks: ~50% chance of normal survival
  • ≥25 weeks: Odds favor normal survival 5

Common Pitfalls and Caveats

1. Incorrect Timing: Scanning too early (<49 days) in asymptomatic women leads to inconclusive results and unnecessary follow-up scans 1

2. Misdiagnosis of Non-Viability: Strict adherence to established criteria (CRL >5mm or MSD ≥16mm with absent cardiac activity) is essential to avoid false diagnosis of pregnancy failure 4

3. Missing Ectopic Pregnancy: Women with pain, bleeding, or history of ectopic pregnancy require earlier assessment regardless of gestational age 1

4. Overreliance on Single Parameters: Multiple parameters (maternal age, gestational sac diameter, and serum progesterone) provide better prediction of pregnancy viability than any single measurement 6

5. Inappropriate Monitoring: Antenatal testing in low-risk women has potential to cause iatrogenic prematurity due to false-positive results 2