Can a day 7 frozen blastocyst be transferred on day 5 or 6 of the menstrual cycle?

Day 7 Blastocyst Transfer Timing

A day 7 frozen blastocyst should be transferred in a frozen embryo transfer (FET) cycle on the appropriate day of progesterone exposure (typically day 5 or 6 of progesterone, not day 5 or 6 of the menstrual cycle), and single embryo transfer must be performed regardless of the embryo's slower development. 1

Understanding the Question Context

The question appears to conflate two different timing concepts that need clarification:

• Day 7 blastocyst refers to when the embryo reached the blastocyst stage during culture (7 days post-fertilization) 2, 3
• Day 5 or 6 of transfer in FET cycles refers to the day of progesterone exposure that matches endometrial receptivity, not the menstrual cycle day 4, 5

Vitrification and Transfer Protocol for Day 7 Blastocysts

Day 7 blastocysts should be vitrified (frozen) and transferred in a subsequent FET cycle, not in fresh cycles. 1, 4

• Vitrification is the preferred cryopreservation method for day 7 blastocysts 1
• Fresh transfer of slowly developing embryos results in significantly worse outcomes due to embryo-endometrial asynchrony 5
• When only morula/compacting embryos are available by Day 5, extending culture until fully expanded blastocysts are achieved on Day 6-7 and performing subsequent FET results in significantly higher live birth rates (20.5% vs 1.8% for fresh morula transfer, P < 0.001) 5

Reproductive Potential of Day 7 Blastocysts

Day 7 blastocysts have lower but non-negligible reproductive potential:

• Euploidy rates decrease with delayed blastulation: 67.7% for Day 5,52.1% for Day 6, and 43.1% for Day 7 blastocysts (Day 5 vs Day 7: OR 0.56,95% CI 0.46-0.69, P < 0.001) 2
• Clinical pregnancy rates for euploid Day 7 embryos range from 16.7% to 52.6% 6
• Live birth rates for euploid Day 7 embryos range from 21.5% to 46% 6
• Sustained implantation rate after euploid Day 7 single embryo transfer is 52.6% (95% CI 35.8-69.0), which is lower than Day 5 (68.9%) and Day 6 (66.8%) but not statistically significantly different 2

Mandatory Single Embryo Transfer Protocol

Single embryo transfer (SET) must be applied for Day 7 blastocysts with no exceptions. 1

• SET should be practiced in vitrified-warmed blastocyst transfer cycles regardless of blastocyst quality, including Day 7 embryos 1, 7
• The decision to perform double embryo transfer should never be based on the number of previous unsuccessful treatments, duration of infertility, or embryo quality 1, 7
• This recommendation prioritizes avoiding multiple pregnancy complications, which include 30-fold higher multiple pregnancy rates, increased pre-eclampsia, gestational diabetes, emergency cesarean section, and preterm labor 8

Clinical Significance and Patient Counseling

Culturing embryos to Day 7 increases the pool of transferrable embryos, particularly for patients who would otherwise have no usable embryos. 2, 3

• Day 7 blastocysts comprise approximately 5-10% of usable blastocysts 3, 6
• This is particularly valuable for older patients (>35 years), whose embryos take longer to blastulate and are more susceptible to cycle cancellation 2
• Trophectoderm biopsy should include 5-8 cells for preimplantation genetic testing cycles, consistent with general PGT standards 1, 7

Common Pitfalls to Avoid

• Do not perform fresh transfer of Day 7 blastocysts - this creates embryo-endometrial asynchrony and significantly reduces success rates 5
• Do not transfer two Day 7 embryos thinking the lower quality justifies double transfer - this violates evidence-based guidelines and increases multiple pregnancy risks without improving cumulative live birth rates 1, 7
• Do not discard Day 7 blastocysts prematurely - they represent viable embryos that can result in healthy live births, especially for patients with limited embryo cohorts 2, 3