When is the first ultrasound performed after Frozen Embryo Transfer (FET)?

First Ultrasound After Frozen Embryo Transfer (FET)

The first ultrasound after a frozen embryo transfer (FET) is typically performed at 6-7 weeks of gestational age to confirm an intrauterine pregnancy with cardiac activity. This timing allows for definitive visualization of key developmental milestones that confirm a viable pregnancy.

Timing of First Ultrasound After FET

Standard Timeline

• Timing: 6-7 weeks gestational age (approximately 4-5 weeks after embryo transfer)
• Rationale: This timing allows for visualization of:
  • Gestational sac (visible at ~5 weeks)
  • Yolk sac (visible at ~5½ weeks)
  • Embryo with cardiac activity (visible at ~6 weeks) 1

Developmental Milestones Visible by Ultrasound

1. ~5 weeks GA: Gestational sac appears as a round/oval fluid collection surrounded by hyperechoic trophoblastic tissue
2. ~5½ weeks GA: Yolk sac becomes visible as a thin-rimmed circular structure within the gestational sac
3. ~6 weeks GA: Embryo with cardiac activity can be visualized
4. ~7 weeks GA: Amnion becomes visible 1

Clinical Considerations for Timing

Benefits of Waiting Until 6-7 Weeks

• Definitive diagnosis: Cardiac activity can be definitively assessed
• Reduced anxiety: Decreases likelihood of inconclusive findings that would require follow-up scans
• Complete assessment: Allows evaluation of:
  • Pregnancy location (intrauterine vs. ectopic)
  • Viability (presence of cardiac activity)
  • Number of embryos/fetuses
  • Chorionicity in case of multiples

Earlier Ultrasound Indications

Earlier ultrasounds (before 6 weeks) may be warranted in specific situations:

• Symptoms suggesting ectopic pregnancy
• Vaginal bleeding or pelvic pain
• History of ectopic pregnancy
• Risk factors for heterotopic pregnancy (especially with IVF) 2

Technical Aspects

Ultrasound Approach

• Transvaginal ultrasound (TVUS) is preferred for early pregnancy assessment due to superior resolution
• Transabdominal ultrasound may be used as a complementary approach if needed 1

Key Elements to Assess

1. Gestational sac: Location, size, shape
2. Yolk sac: Presence, size, shape
3. Embryo: Presence, crown-rump length (CRL)
4. Cardiac activity: Presence, heart rate
5. Multiple gestation: If present, determine chorionicity and amnionicity 1

Special Considerations

Multiple Gestations

• For multiple gestations, determining chorionicity is critical at the first ultrasound
• For monochorionic twins, additional monitoring will be required starting at 16 weeks for complications such as twin-twin transfusion syndrome 1

Potential Pitfalls

• Too early scanning: May lead to inconclusive results and unnecessary anxiety
• Misinterpretation: Intracavitary fluid can mimic a gestational sac (previously called "pseudosac") 1
• Heterotopic pregnancy: Can be missed if focus is only on intrauterine findings 2

Follow-up Ultrasound Recommendations

After the initial 6-7 week ultrasound:

• Normal singleton pregnancy: Next ultrasound typically at 11-14 weeks for NT assessment
• Monochorionic twins: Begin ultrasound surveillance at 16 weeks and continue every 2 weeks until delivery 1
• Dichorionic twins: Less frequent monitoring than monochorionic twins, but still requires specialized care 3

By scheduling the first ultrasound after FET at 6-7 weeks gestational age, clinicians can provide the most definitive assessment of pregnancy location, viability, and number of embryos while minimizing the need for repeat scans due to inconclusive findings.