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Research Article: Impact of Day-3 embryo cell number on pregnancy, obstetric and perinatal outcomes in frozen-thawed single blastocyst transfer cycles

Date Published: 2026-06-22

Abstract:
To evaluate the impact of Day-3 embryo cell number on pregnancy, obstetric, and perinatal outcomes after frozen-thawed single blastocyst transfer (SBT). This retrospective cohort study included 6437 frozen–thawed SBT cycles performed between 2016 and 2024 at a large reproductive center. Cycles were stratified by Day-3 cell number into six groups (? 5, 6, 7, 8, 9, and ? 10 cells). Pregnancy outcomes included clinical pregnancy, ongoing pregnancy, live birth, and early miscarriage rates. Obstetric and perinatal outcomes were assessed in 3191 singleton live-birth cycles. Multivariable logistic regression, stratified sensitivity analyses, and propensity score matching (PSM) were performed to adjust for relevant covariates. Compared with the 8-cell group, blastocysts derived from Day-3 embryos with ?5 or 6 cells were associated with significantly lower odds of live birth (?5-cell: aOR 0.69, 95% CI 0.56–0.85; 6-cell: aOR 0.76, 95% CI 0.60–0.95), clinical pregnancy, and ongoing pregnancy after multivariable adjustment. These associations were further confirmed after PSM. In stratified analyses, the adverse associations observed for the ?5-cell group remained consistent across subgroups defined by blastocyst quality or blastocyst day. In contrast, the associations observed in the 6-cell group remained directionally similar to those in the overall analysis, although they were no longer statistically significant among Day-6 blastocysts. Regarding obstetric and perinatal outcomes, neither the ?5-cell nor the 6-cell group showed significant differences in gestational age, preterm birth, cesarean delivery, major obstetric complications, neonatal sex, birth weight, or birth defects compared with the 8-cell group. Overall, blastocysts derived from Day-3 embryos with ?5 or 6 cells were associated with reduced pregnancy outcomes after frozen-thawed SBT. However, among singleton live births, no adverse associations with obstetric or perinatal outcomes were observed in either group. These findings may help inform blastocyst selection and patient counselling during IVF treatment.

Introduction:
The in vitro fertilization (IVF) procedure became the primary treatment for infertility worldwide following the successful birth of the first IVF baby in 1978. During those early decades, embryo transfer was mainly performed at the cleavage stage, and the embryos were evaluated based on morphological features such as blastomere number, degree of fragmentation, and cell symmetry ( 1 ). The blastomere number or cell number—the number of cells formed during cleavage divisions of the fertilized oocyte—reflects the…

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