Day-5 versus Day-6 Blastocyst Transfer: Does Embryo Developmental Speed Independently Predict Live Birth?

Background: Blastocysts that reach the expanded or hatching stage by day 5 of culture are conventionally preferred for transfer over those reaching equivalent morphology only on day 6, but whether developmental day itself independently predicts live birth, beyond its correlation with embryo morphology and chromosomal status, remains debated. 

Objective: This narrative review synthesizes the available evidence on whether day of blastocyst development (day 5 vs. day 6) independently predicts live birth following IVF/ICSI, after accounting for embryo quality and, where possible, ploidy status. Methods: A structured search of academic databases and search engines was conducted for primary cohort studies and systematic reviews/meta-analyses comparing live birth outcomes between day-5 and day-6 blastocyst transfers, with attention to studies that adjusted for or restricted analysis to euploid or morphologically graded embryos. Seven primary cohort studies and two systematic reviews/meta-analyses met inclusion criteria. 

Results: Across nearly all included studies, day-6 blastocyst transfer was associated with a significantly lower live birth rate than day-5 transfer, and this association persisted after statistical adjustment for embryo morphology in several cohorts, and after restricting analysis to euploid blastocysts only in others, including one donor-oocyte cohort that additionally controlled for oocyte age. However, a substantial minority of evidence complicates a simple independent-predictor interpretation: one large cohort found that high-grade day-6 blastocysts achieved higher live birth rates than low-grade day-5 blastocysts, and a time-lapse study restricted to euploid or low-mosaic embryos found that live birth was more directly associated with blastocyst morphology than with developmental kinetics once morphology was accounted for. The two identified meta-analyses both concluded that day-5 transfer is generally favoured, while one explicitly noted that outcomes converge when day-5 blastocysts are poor-quality and day-6 blastocysts are high-quality. Discussion: The evidence is most consistent with developmental day functioning as a partially independent but not dominant predictor of live birth: it retains statistical significance after adjustment for morphology and euploidy in several well-controlled cohorts, but its practical importance is regularly outweighed by blastocyst morphological grade, and, most likely, by the aneuploidy risk that correlates with, but is not fully explained by, slower development. 

Conclusion: Day-6 development is associated with modestly but consistently poorer live birth outcomes even after adjustment for embryo quality in several cohorts, but developmental speed should be interpreted as one prognostic factor among several, not a standalone determinant, and a high-quality day-6 blastocyst should generally be preferred over a poor-quality day-5 blastocyst when a choice must be made.

The extension of embryo culture to the blastocyst stage has become standard practice in many IVF/ICSI programmes, allowing embryologists to select embryos with demonstrated developmental competence rather than relying solely on early cleavage-stage morphology. Most human blastocysts reach the expanded or hatching stage suitable for biopsy or transfer by day 5 of culture, but a substantial minority do so only on day 6, and a smaller proportion on day 7. Clinical practice has long favoured day-5 blastocysts for transfer when available, reflecting an accumulated body of observational evidence linking slower blastulation to lower implantation and live birth rates.

What remains less settled is the mechanism, and therefore the clinical weight that developmental day should be given relative to other, more direct measures of embryo competence. Slower-developing blastocysts are known to have both poorer morphological grades (smaller inner cell mass, lower-quality trophectoderm) and a higher rate of chromosomal aneuploidy than same-cohort blastocysts reaching full expansion by day 5. This raises the central question addressed by this review: does day of blastocyst development predict live birth independently of embryo morphology and ploidy status, or does its apparent effect largely reflect the fact that day-6 blastocysts are, on average, lower-quality and more often aneuploid than day-5 blastocysts from the same cohort? The distinction matters clinically: if developmental day is largely a proxy for quality and ploidy, then a morphologically excellent, euploid day-6 blastocyst should perform as well as an equivalent day-5 blastocysts; if day itself carries additional, independent prognostic information, even quality- and ploidy-matched day-6 blastocysts should be expected to underperform.

This review aims to (a) synthesize the available cohort and meta-analytic evidence comparing live birth outcomes after day-5 versus day-6 blastocyst transfer; (b) specifically evaluate studies that adjusted for embryo morphology or restricted analysis to euploid blastocysts, in order to isolate any independent contribution of developmental day; and (c) identify the clinical and counter-evidence suggesting that blastocyst quality can offset or outweigh the day effect.

This is a narrative review rather than a formally registered PRISMA systematic review. A structured search was conducted across academic search engines combining terms related to the exposure ("day 5," "day 6," "blastulation," "developmental speed," "blastocyst") with terms related to the outcome ("live birth," "live birth rate," "implantation," "clinical pregnancy") and terms related to quality/ploidy control ("euploid," "morphology," "grade," "PGT-A," "independent predictor"). Reference lists of retrieved systematic reviews were also hand-searched.

Inclusion criteria comprised studies that (a) reported primary empirical data comparing day-5 and day-6 (or later) blastocyst transfer cycles, or were themselves systematic reviews/meta-analyses; (b) reported live birth rate (or, where unavailable, clinical or ongoing pregnancy rate as a closely related outcome) as an outcome; and (c) either statistically adjusted for embryo morphology/quality, or restricted their analysis to euploid or PGT-tested blastocysts, allowing at least partial isolation of a day-independent effect. Studies reporting only laboratory endpoints (e.g., blastocyst formation rate) without any pregnancy or live-birth outcome, and studies focused exclusively on day 7 blastocysts without a day-5/day-6 comparison, were excluded.

Seven primary cohort studies and two systematic reviews/meta-analyses met inclusion criteria and form the basis of the synthesis below (Table 1). For each study, the following were extracted: authorship and year, design, sample, the degree of quality/ploidy control applied, and the key live-birth-related finding. Given heterogeneity in population, embryo transfer strategy (single vs. multiple), and the specific adjustment or restriction strategy used, a narrative rather than a meta-analytic synthesis was undertaken, organized around two evidence strands: (a) studies finding day-6 transfer independently associated with lower live birth even after adjustment for morphology or euploidy, and (b) studies and meta-analytic subgroup findings suggesting that blastocyst quality can offset or explain much of the apparent day effect.

Evidence for an independent effect of developmental day

Several well-controlled cohorts found that day-6 development remained associated with lower live birth rates even after statistical adjustment for embryo quality. In a cohort of 669 vitrified-warmed blastocysts, delayed blastulation and multinucleation were independently associated with lower live birth rates in multivariable analysis, and blastocyst expansion grade after warming predicted outcome independent of inner cell mass or trophectoderm score (Desai et al., 2016). In a cohort restricted specifically to euploid blastocysts, day-5 transfer was associated with a significantly higher live birth rate than day-6 transfer even after adjusting for morphologic grade, maternal age, body mass index, endometrial thickness, number of available euploid embryos, and frozen embryo transfer protocol (adjusted odds ratio 1.5, 95% CI 1.1–2.1) (Irani et al., 2018), directly addressing the concern that the day effect might simply reflect a higher aneuploidy rate among day-6 embryos.

A retrospective cohort of 327 vitrified-warmed blastocysts similarly found a significantly higher live birth rate with day-5 transfer, with an adjusted odds ratio for delivery of 2.94 favouring day 5 after adjustment for confounding factors and stratification by developmental stage (Tubbing, Shaw-Jackson, Ameye, Colin, Rozenberg & Autin, 2018). Perhaps the strongest design for isolating a day-independent effect comes from a multi-center oocyte-donation cohort of 1,840 frozen embryo transfers, in which recipient and donor age, an important confounder in autologous cycles, was effectively controlled by the use of young, screened oocyte donors; day-5 transfer remained an independent predictor of live birth (adjusted odds ratio 2.26, 95% CI 1.19–4.28) regardless of embryo quality (Yerushalmi et al., 2021). A large PGT cohort restricted to euploid blastocysts similarly identified developmental day, alongside blastocyst number, endometrial preparation protocol, and blastocyst quality, as an independent predictor of live birth in multivariable regression, and used these factors to build a predictive nomogram (Yin, Li, Sun, Yao, Cui, Zhang & Zhang, 2024).

Evidence that quality can offset or explain the day effect

Other well-designed studies complicate a simple "day 6 is independently worse" narrative. In a large retrospective cohort explicitly designed to compare blastocysts of discordant day and quality, high-grade day-6 blastocysts achieved a significantly higher live birth rate than low-grade day-5 blastocysts (50.4% vs. 40.7%, adjusted odds ratio 1.54, 95% CI 1.05–2.26); notably, high-grade day-6 blastocysts also had a higher euploidy rate than low-grade day-5 blastocysts in this cohort's PGT-A sub-analysis, suggesting that morphological grade may better capture developmental competence than day alone (Shi, Zhou, Chen, Xue & Shi, 2023).

A time-lapse monitoring study restricted to euploid or low-level mosaic single-embryo transfers found that day-6 status was associated with lower live birth in an unadjusted analysis (adjusted odds ratio 0.341, 95% CI 0.169–0.685), but further analysis indicated that live birth probability was more directly associated with blastocyst morphology, specifically inner cell mass and trophectoderm grade, than with developmental kinetics themselves once dysmorphisms and morphology were accounted for (Chen, Lee, Huang et al., 2023). This finding is consistent with an interpretation in which developmental day functions largely as a marker for underlying morphological and kinetic quality rather than as an independent causal factor in its own right.

Systematic reviews and meta-analyses

The most comprehensive synthesis to date, a systematic review and meta-analysis of 47 studies spanning both fresh and frozen blastocyst transfer cycles, concluded that day-5 blastocyst transfer is associated with significantly higher clinical pregnancy and live birth rates than day-6 transfer in both settings, and that this advantage holds regardless of embryo quality across the pooled literature (Bourdon et al., 2019). A more recent meta-analysis, incorporating euploid-only subgroups and perinatal outcomes, reached a similar overall conclusion, that day-5 blastocysts are more likely than day-6 to result in pregnancy and live birth, with and without preimplantation genetic testing, and that neonatal outcomes do not differ meaningfully by developmental day, but explicitly noted that the clinical outcomes of poor-quality day-5 blastocysts and high-quality day-6 blastocysts were statistically similar, concluding that developmental speed and blastocyst quality jointly, rather than developmental speed alone, determine outcome (Hou, Wang, Wen, Ma, Lu, Fan & Zhao, 2026).

Table 1 summarizes the design, sample, quality/ploidy control, and key live-birth-related finding of the nine studies included in this review.

Table 1: Characteristics of Included Studies on Day-5 versus Day-6 Blastocyst Transfer and Live Birth

Note: aOR = adjusted odds ratio; BMI = body mass index; D5/D6 = day 5/day 6; FET = frozen embryo transfer; ICM = inner cell mass; PGT-A = preimplantation genetic testing for aneuploidy; TE = trophectoderm.

The evidence reviewed here supports a nuanced answer to the review's central question: developmental day carries a statistically detectable, partially independent association with live birth beyond embryo morphology and, in several cohorts, beyond euploidy status, but this association is neither large nor dominant relative to morphological grade, and is regularly overridden by it in head-to-head comparisons. The strongest support for a genuinely independent day effect comes from three design features seen across the included studies: adjustment for morphological grade while the day effect persisted (Desai et al., 2016; Tubbing et al., 2018), restriction to euploid blastocysts only while the day effect persisted (Irani et al., 2018; Yin et al., 2024), and control of oocyte-age-related confounding through a donor-oocyte model while the day effect persisted (Yerushalmi et al., 2021). Taken together, these designs address the two most obvious alternative explanations, that day 6 is simply a proxy for poorer morphology or for higher aneuploidy, and in each case the day effect remained statistically significant after that specific confound was addressed.

At the same time, the counter-evidence is not marginal. The finding that high-grade day-6 blastocysts outperform low-grade day-5 blastocysts (Shi et al., 2023), replicated in the quality-stratified subgroup of the most recent meta-analysis (Hou et al., 2026), demonstrates that morphological grade carries more prognostic weight than developmental day when the two are in conflict. Similarly, the time-lapse study's finding that live birth tracked morphology more closely than developmental kinetics once both were modelled (Chen et al., 2023) suggests that at least part of the apparent "day" effect in less granular analyses may be statistical confounding by unmeasured or incompletely adjusted morphology, even in some studies that nominally adjusted for grade using coarser categorical scoring systems.

A biologically plausible synthesis is that developmental day, morphological grade, and euploidy status are three correlated but imperfectly overlapping markers of the same underlying construct, overall blastocyst developmental competence, arising from a shared cause (the oocyte's and early embryo's intrinsic quality) rather than day exerting a separate, direct causal effect on implantation potential. Under this view, day should be expected to retain some independent statistical association in any given regression model, because no morphological or genetic assessment captures embryo competence perfectly, but its practical clinical importance should be, and empirically is, secondary to direct measures of morphology and, where available, ploidy. This is consistent with current clinical practice recommendations that prioritise blastocyst grade and euploidy status over developmental day alone when multiple embryos are available for transfer.

Several methodological limitations affect the strength of these conclusions. Nearly all included studies are retrospective cohorts subject to potential selection bias, since the decision to culture an embryo to day 6 (rather than transfer or discard on day 5) is not randomised and may itself be influenced by embryologists' assessment of embryo quality at earlier time points. Morphological grading systems, while broadly similar across centres, are not perfectly standardised, and coarse categorical grading (e.g., "good" vs. "poor") may not fully capture the continuous variation in embryo quality that could otherwise explain residual day effects. Euploidy status, where reported, was determined by trophectoderm biopsy and next-generation sequencing, methods subject to a small but non-zero misclassification rate, including mosaicism, that could bias adjusted estimates in either direction. Finally, none of the included primary studies were randomised, and randomisation of developmental day is not ethically or practically feasible, meaning causal inference in this literature will always rest on observational adjustment rather than experimental control.

Priorities for future research include: (a) larger cohorts using continuous or finer-grained morphological scoring (rather than coarse good/poor categories) combined with euploidy status, to more precisely quantify how much residual, unexplained variance developmental day accounts for once quality and ploidy are modelled together; (b) time-lapse and morphokinetic studies that can decompose "day of blastulation" into its component kinetic parameters (e.g., timing of specific cleavage and compaction events) to identify which, if any, specific kinetic markers carry independent prognostic value; (c) studies in donor-oocyte or similarly age-controlled models, replicating the design of Yerushalmi et al. (2021), to further isolate day effects from age-related aneuploidy; and (d) direct, prospective comparison of a decision rule prioritising day over grade against one prioritising grade over day, to establish which selection strategy produces better real-world live birth outcomes when clinicians must choose among multiple available blastocysts.

Day-6 blastocyst development is consistently associated with lower live birth rates than day-5 development across the reviewed literature, and this association persists in several well-controlled cohorts after adjustment for embryo morphology, restriction to euploid blastocysts, or control of oocyte-age-related confounding through a donor-oocyte model, supporting a genuine, if modest, independent contribution of developmental speed to live birth prediction. However, the practical clinical weight of developmental day is consistently secondary to blastocyst morphological grade: high-quality day-6 blastocysts outperform low-quality day-5 blastocysts in direct comparison, and clinical outcomes converge once morphology is matched between groups in the most recent meta-analytic evidence. Developmental speed should therefore be treated as one contributory prognostic factor among several, rather than as a standalone determinant of live birth, and embryo selection decisions should continue to prioritise direct assessment of blastocyst morphology and, where available, ploidy status over developmental day alone.

This manuscript was prepared as a structured draft to support submission and is intended for the author's review before journal submission. Before submission: (1) independently verify every citation against its primary source, and complete the full author list for Yerushalmi et al. (2021), abbreviated here as "et al."; (2) search for any more recent studies or meta-analyses published after this draft was prepared, given the fast-moving nature of this literature; (3) consider whether a full PRISMA systematic review (with protocol registration, dual independent screening, and a PRISMA flow diagram) is required by the target journal, as this narrative review does not claim that level of methodological rigor; and (4) adapt formatting, length, and citation style to the target journal's author guidelines.