What is expected to be visible on an ultrasound at the 3rd week post Frozen Embryo Transfer (FET)?

Ultrasound Findings at 3 Weeks Post Frozen Embryo Transfer

At 3 weeks post frozen embryo transfer (FET), an ultrasound should show a gestational sac with a visible yolk sac and embryo with cardiac activity, as this corresponds to approximately 6 weeks gestational age. 1, 2

Expected Findings Based on Timing

• Gestational age calculation: 3 weeks post-FET typically corresponds to approximately 6 weeks gestational age (counting from last menstrual period in a natural cycle)

  • Day of FET is considered equivalent to ovulation (approximately 2 weeks after LMP)
  • Add 3 weeks post-transfer to get ~5-6 weeks gestational age

• Structures that should be visible:

  • Gestational sac: Typically visible from 5 weeks GA as a round or oval fluid collection surrounded by hyperechoic trophoblastic tissue 1
  • Yolk sac: Should be visible at 5½ weeks GA as a thin-rimmed circular structure eccentrically located within the gestational sac 1, 2
  • Embryo: Should be visible with cardiac activity at approximately 6 weeks GA 2
  • Cardiac activity: Should be measurable using M-mode or cine clip 2

Important Measurements and Assessment

• Mean sac diameter (MSD): Calculated as (Length + Width + Height)/3 2
• Crown-rump length (CRL): Most accurate measurement for dating once the embryo is visible 2
• Cardiac activity: Normal range at 6-7 weeks is 100-120 beats per minute 2
  • <100 bpm at 6.2 weeks or earlier is concerning
  • <120 bpm at 6.3-7.0 weeks is concerning

Location and Implantation Assessment

• The gestational sac should be located within the upper 2/3 of the uterus in a normal pregnancy 1
• The intrauterine location should be confirmed by visualization of the gestational sac surrounded by endometrium 1
• The presence of a yolk sac within an intrauterine fluid collection is incontrovertible evidence of a definite intrauterine pregnancy 1

Multiple Gestation Considerations

• If multiple embryos were transferred, assessment should include:
  • Number of gestational sacs visible 1, 2
  • Chorionicity determination (best assessed before 10 weeks) 1
  • Amnionicity determination (intertwin membrane typically visible by 10 weeks) 1
  • Crown-rump length of each embryo to assess for discordance 1

Common Pitfalls to Avoid

• Misinterpreting intracavitary fluid as a gestational sac 1, 2
• Incorrect dating based on measurements 2
• Missing multiple gestations 1, 2
• Failure to document cardiac activity properly 2

Follow-up Recommendations

If expected findings are not visualized at 3 weeks post-FET:

• A repeat high-resolution ultrasound should be performed within 7-14 days 2
• Serial hCG measurements may be needed to assess for appropriate rise
• The presence of a triple-line endometrial pattern is associated with better outcomes in FET cycles 3

The transvaginal ultrasound approach is superior for early pregnancy assessment before 10 weeks gestation, especially in obese patients and those with retroverted uteri 2.