Where and how is pregnancy diagnosis and viability assessed?

Pregnancy Diagnosis and Viability Assessment

Transvaginal ultrasound (TVUS) is the cornerstone for diagnosing pregnancy location and assessing viability, with specific ultrasound markers visualized at different gestational ages providing critical information about pregnancy status and prognosis.

Pregnancy Diagnosis and Location

Clinical Assessment

• Initial assessment includes:
  • Positive pregnancy test (serum or urine hCG)
  • Menstrual history to estimate gestational age
  • Symptoms (amenorrhea, morning sickness, breast tenderness)

However, clinical examination alone is insufficient for reliable pregnancy diagnosis and should always be confirmed with laboratory testing 1.

Ultrasound Assessment

Transvaginal ultrasound is the primary modality for determining pregnancy location and viability, with specific findings appearing at predictable gestational ages:

1. Gestational Sac (GS)

   • Visualized at approximately 5 weeks gestational age
   • Round or oval fluid collection within the uterus
   • Mean sac diameter (MSD) = (Length + Width + Height)/3 2
   • Used for dating before embryo is visible

2. Yolk Sac (YS)

   • Thin-rimmed circular structure within the gestational sac
   • Visualized at approximately 5½ weeks
   • Confirms definite intrauterine pregnancy
   • Typically ≤6 mm in diameter 2

3. Embryo

   • Visualized at approximately 6 weeks
   • Crown-rump length (CRL) is measured as the greatest dimension
   • Used for accurate pregnancy dating 2

4. Cardiac Activity

   • Rhythmic pulsations in embryo visualized at approximately 6 weeks
   • Should be documented with M-mode or cine clip
   • Measured in beats per minute (bpm) 2

5. Amnion

   • Thin membrane surrounding embryo
   • Visualized at approximately 7 weeks
   • Yolk sac always remains outside the amniotic cavity 2

Pregnancy Viability Assessment

Ultrasound Criteria for Viable Pregnancy

A viable intrauterine pregnancy is characterized by:

• Appropriately sized gestational sac for gestational age
• Presence of yolk sac
• Embryo with cardiac activity
• Appropriate growth between serial ultrasounds

Criteria for Early Pregnancy Loss (EPL)

The Society of Radiologists in Ultrasound recommends using the term "Early Pregnancy Loss" rather than "pregnancy failure" or "nonviable pregnancy" 2. EPL is diagnosed when:

1. Diagnostic of EPL:

   • Crown-rump length ≥7 mm with no cardiac activity
   • Mean gestational sac diameter ≥25 mm with no embryo
   • Absence of embryo with cardiac activity ≥2 weeks after a scan showing a gestational sac without yolk sac
   • Absence of embryo with cardiac activity ≥11 days after a scan showing a gestational sac with yolk sac 2

2. Concerning for EPL:

   • Irregular gestational sac shape
   • Abnormal size or shape of yolk sac
   • Embryo with crown-rump length <7 mm without cardiac activity 2

Pregnancy of Unknown Location (PUL)

• Defined as positive pregnancy test with no evidence of intrauterine or ectopic pregnancy on ultrasound
• Transient state requiring follow-up
• Most common outcomes: early viable IUP, nonviable IUP, or ectopic pregnancy 2
• Follow-up with serial hCG measurements and repeat ultrasound is essential

Ectopic Pregnancy Assessment

• Always evaluate extrauterine locations when an IUP is not identified
• Key findings include:
  • Extrauterine gestational sac with embryo (100% specific)
  • Tubal ring (extrauterine mass with fluid center and hyperechoic periphery)
  • Nonspecific heterogeneous adnexal mass
  • Free fluid in pelvis (especially if containing echoes) 2
• Common locations: fallopian tube (most common), interstitial, cervical, cesarean section scar

Prediction Models for Pregnancy Viability

Several scoring systems have been developed to predict pregnancy viability:

1. Bottomley Score:

   • Combines clinical history and ultrasound parameters
   • Includes maternal age, bleeding score, gestational age, presence of yolk sac, fetal heart pulsation, yolk sac size, and fetal size
   • Score ≥1 corresponds to >90% probability of viable pregnancy 3

2. Logistic Regression Model:

   • Combines maternal age, gestational sac diameter, and serum progesterone levels
   • Area under curve of 0.85 for predicting viability 4
   • Particularly useful when no embryo is visible on ultrasound

Special Considerations

Gestational Trophoblastic Disease (GTD)

• Ultrasound findings: hyperechoic area in endometrium with multiple cystic spaces (complete molar pregnancy)
• Early first trimester appearance may be variable
• Partial molar pregnancy may show embryo with cystic change in early placenta
• Definitive diagnosis requires histopathological evaluation 2

Safety Considerations

• Avoid pulsed Doppler ultrasound in first trimester when normal IUP is present or suspected
• M-mode is preferred for documenting embryonic cardiac activity
• MRI can be used in second/third trimesters when needed, but gadolinium should not be administered during pregnancy 2
• CT and radioisotope studies should be avoided during pregnancy 2

Common Pitfalls and Caveats

1. Terminology Pitfalls:

   • Avoid terms like "heart motion," "heartbeat," "live," "living," and "viable" in first trimester
   • Use "cardiac activity" or "cardiac motion" instead 2
   • Avoid "viability scan" - use "first trimester ultrasound" or "early pregnancy ultrasound" 2

2. Diagnostic Pitfalls:

   • Pseudogestational sac (intracavitary fluid) can be mistaken for early IUP
   • Corpus luteum can be mistaken for ectopic pregnancy (located in ovary vs. extraovarian)
   • Early molar pregnancy may appear similar to retained products of conception

3. Management Pitfalls:

   • Avoid premature intervention in PUL - follow-up is essential
   • Don't rely on transabdominal ultrasound alone for early pregnancy assessment
   • Ensure histological examination of all products of conception from non-viable pregnancies 2

By following these guidelines for pregnancy diagnosis and viability assessment, clinicians can accurately determine pregnancy location, assess viability, and guide appropriate management to optimize maternal and fetal outcomes.