Factors Affecting Embryo Developmental Pace

Presented at: ASRM Scientific Congress, 2021

Authors: Leah Kaye, Melody Rasouli, Ankita Raman, Forest Garner, Carrie Bedient, Bruce Shapiro

Materials and Methods:

In this prospective observational study, patients underwent routine ovarian stimulation with exogenous gonadotropins. During oocyte collection, each punctured follicle was sonographically measured. Embryos were group-cultured to the blastocyst stage according to follicle diameter. The day of blastulation was recorded for blastocysts of transferable quality (no CC grades). Blastocysts forming after day 5 were considered delayed. Logistic regressions were run to determine if patient age, follicle diameter, and the total number of eggs collected correlated with blastocyst formation on day 5. Because of the potential for biopsy to artificially influence blastulation, separate logistic models were developed for biopsied and non-biopsied blastocysts. P<0.05 was considered significant.


The day of blastulation was recorded for 514 blastocysts derived from measured follicles, 221 of which formed on day 5, 249 on day 6, and 44 on day 7. Two logistic regression models were constructed. The first logistic model was developed predicting day 5 blastulation among the 170 biopsied blastocysts. The only significant predictor of day 5 blastulation in this model was follicle diameter (P=0.0038). The second logistic model was developed for 344 blastocysts that were not biopsied. In this second model, the only significant predictor of day 5 blastulation was, again, follicle diameter (P=0.0067). In both models, increasing follicle diameter corresponded with increased risk of delayed blastulation (day 6 or day 7 blastulation).


Regardless of biopsy for PGT-A, larger follicles were significantly associated with delayed blastulation. Patient age and the number of eggs collected were not significant predictors of delayed blastulation.

Impact Statement:

Previous studies have found that large follicles are associated with increased chance of obtaining blastocysts. The current findings suggest those blastocysts derived from large follicles are associated with delayed (day 6 or day 7) blastulation. This may hold implications regarding oogenesis and ovarian stimulation protocols, particularly in fresh transfer cycles.

  Follicle diameter at retrieval
≤12.5mm 13mm to 18.5mm 19mm to 24.5mm ≥25mm
Day 5 blastulation 37 (54.4%) 99


76 (37.4%) 9 (19.6%)
Day 6 blastulation 24


85 (43.2%) 113 (55.7%) 27 (58.7%)
Day 7 blastulation 7


14 (6.6%) 14 (6.9%) 10 (21.7%)
Total 68 (100%) 198 (100%) 203 (100%) 46 (100%)