Day 5 versus Day 6 Blastocyst Embryo Transfer

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Day 5 versus day 6 blastocyst embryo transfer


In this study, day 5 blastocyst embryo transfers resulted in significantly higher rates of clinical pregnancy (34.2% vs. 20.2%) and live birth (26.9% vs 14.8%) compared to day 6 blastocysts transfers in FET cycles, with a potential decrease in miscarriage rates (7.7% vs. 13.5%, P=0.05).


Studies comparing fresh embryo transfer show an advantage of day 5 transfer compared to day 6. However in frozen embryo transfer (FET) cycles, results so far have been conflicting.

In 2010 a meta-analysis showed similar clinical pregnancy and live birth rates among day 6 and day 5 cyropreserved embryos at identical stages of development.

A more recent meta-analysis however suggested that both fresh and frozen day 5 blastocyst transfers had a higher clinical pregnancy and live birth rate compared to day 6 transfer.

By and large these studies were not originally designed to compare day 5 to day 6 vitrified embryos, featuring a variety of patients, stimulation protocols and cryopreservation methods.


To compare the clinical outcomes of day 5 and day 6 vitrified embryo transfer.


A total of 783 women participated in the study with oocytes donated from a single medical center.

Oocyte donors were aged 20-32 years old, with adequate ovarian reserve (AMH > 2, AMH > 16). A single stimulation protocol (GnRH antagonist plus agonist trigger) was used across all donors to minimise potential bias. Oocytes were then fertilised (sperm with concentration > 1million/ml), cultured and vitrified on day 5 or 6, using the SAGE® Vitrification Kit. Only good quality blastocysts (grade ≥ 3BB) were selected for transfer.

Endometrial preparation was carried out via hormone replacement therapy (HRT) or under natural cycle (NC). Luteal (progesterone) support was administered for 5 days prior to transfer of embryo, following 2-4 hours of thawing using the SAGE® Vitrification warming Kit.

Ultrasound confirmation of fetal heart beat within the first trimester defined clinical pregnancy rate (CPR) while loss of fetal heart rate prior to 20 weeks gestation was recorded as a missed abortion (miscarriage).


Of the 1898 FETs, 58 cycles were excluded from analysis for breach of study design or lack of data.

In total, 1840 blastocyst transfers (day 5 = 1180, day 6 = 660) were included in the final analysis, and 1389/1840 transfers (day 5 = 915, day 6 = 474) in the live birth rate (LBR).

Initial analysis of baseline characteristics, comparing day 5 to day 6 embryos, showed no significant differences in donor age, recipient age, sperm quality or mean number of oocytes per patien.

However a statistically significant increase in cleavage (83.8% vs. 81.8%, P=0.0008) and fertilisation rates (85.1% vs. 83.6%, P=0.007) was seen among day 5 embryos compared to day 6 embryos.

This statistically significant difference between the 2 groups was also seen in the mean weighted day 3 embryo scores (2.10 vs. 2.02), blastocyst development rate (37.0% vs. 12.34%), freezing (45.6% vs. 37.8%) and survival rates (94.9% vs. 90.6%).

Overall day 5 blastocysts had significantly higher CPR (34.2% vs. 20.2%) and LBR (26.9% vs 14.8%) compared to day 6 blastocysts following transfer, with potentially decreased rates of abortion (7.7% vs. 13.5%, P=0.05). This trend remained consistent even when restricting analysis to only first embryo transfer results in patients.

Further comparison of day 5 to day 6 outcomes, according to embryo quality (Top, Good, Fair) showed a similar trend in all embryo quality groups, with higher CPR and LBR among day 5 blastocysts.

This resulted in a significantly lower mean number of cycles to live birth (1.83 vs 2.39) and mean number of days to pregnancy leading to live birth (76.3 vs 123.2) for day 5 embryos compared to day 6.

Advanced statistical analysis (multivariate logistic regression) confirmed that day 5 blastocyst transfer had 1.91 times improved odds of clinical pregnancy compared to day 6. Similarly top quality and good quality embryos, compared to fair quality embryos, showed improved odds of 1.57 and 1.39 respectively.

Interestingly increased patient age only showed a minor decrease in the overall odds (Odds ratio = 0.96) of clinical pregnancy per each year in age difference.

Overall final analysis confirmed that day 5 blastocysts were 2.26 times more likely to result in a live birth compared to day 6 blastocyst transfers.


  1. Retrospective study design limited authors ability to collect more relevant data.
  2. Embryo euploidy was not confirmed by PGT.
  3. Lack of sperm count, morphology and motility data.
  4. Older study population (average age 43).


No external funding was provided for this study.


A fertilised embryo that has developed an inner cell mass and outer layer (trophoblast) some time from day 4 onwards.

Preservation of an embryo at extremely low temperatures.

Second half of the menstrual cycle, from ovulation to the start of menstruation.


Statistical analysis combing the results of multiple scientific studies.

Unfertilised immature / mature egg.

Rapid freezing an embryo to avoid formation of ice crystals.

Similar studies

Li Y X, et al. (2020). Pregnancy outcomes after day 5 versus day 6 blastocyst-stage embryo transfer: A systematic review and meta-analysis.

Ferreux L, et al. (2018). Live birth rate following frozen-thawed blastocyst transfer is higher with blastocysts expanded on Day 5 than on Day 6.

Tubbing A and Shaw-Jackson C, (2018). Increased live births after day 5 versus day 6 transfers of vitrified-warmed blastocysts.

Yang H, et al. (2016). Comparison of differences in development potentials between frozen-thawed D5 and D6 blastocysts and their relationship with pregnancy outcomes.

El-Toukhy T, et al. (2011). Delayed blastocyst development does not influence the outcome of frozen-thawed transfer cycles.


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