Abstract
<title>Abstract</title> <p> To identify factors influencing Day 3 (D3) embryo partial compaction (PCM) in IVF/ICSI fresh cycles, and to evaluate the blastocyst developmental potential, clinical pregnancy outcomes and neonatal safety of embryos with varying compaction degrees and cell numbers, we conducted this retrospective cohort study including 1,917 patients undergoing IVF/ICSI fresh cycles between August 2020 and December 2025. Patients were divided into PCM ( <italic>n</italic> = 434) and non-PCM ( <italic>n</italic> = 1,483) groups. Independent factors for PCM were identified using logistic regression. Embryos were classified into six mutually exclusive groups: fully compacted; PCM with ≤ 7, 8, or ≥ 9 cells; 8-cell grade I/II (reference); and ≥ 6-cell non-8-cell good-quality embryos. Blastocyst development, pregnancy and neonatal outcomes were compared across groups. Overall PCM incidence was 22.64%. Multivariate analysis identified elevated BMI (OR = 1.044, 95% CI: 1.017–1.071, <italic>P</italic> = 0.001) and higher oocyte yield (OR = 1.053, 95% CI: 1.030–1.076, <italic>P</italic> < 0.001) as independent risk factors, while longer infertility duration (OR = 0.946, 95% CI: 0.906–0.988, <italic>P</italic> = 0.012) and ICSI (OR = 0.751, 95% CI: 0.565–0.998, <italic>P</italic> = 0.048) were protective. Fully compacted embryos performed comparably to 8-cell grade I/II embryos ( <italic>P</italic> > 0.05), but significantly outperformed PCM-≤7 cell and ≥ 6-cell non-8-cell groups ( <italic>P</italic> < 0.001). Notably, PCM-8 cell embryos achieved the highest blastocyst formation (90.9%) and usable blastocyst rates (83.3%). In double embryo transfers, the fully compacted/PCM = 8 group had the highest clinical pregnancy rate (61.76%) and implantation rate; the PCM ≥ 9 group showed a significantly elevated miscarriage rate; cycles containing 8-cell embryos had higher live birth rates than non-8-cell cycles ( <italic>P</italic> = 0.047). No significant differences were observed in neonatal outcomes across the six groups ( <italic>P</italic> > 0.05). D3 embryo PCM is a common morphological variant regulated by maternal BMI, ovarian response, infertility duration and fertilization method, representing a potential benign developmental variation. Fully compacted embryos demonstrate developmental competence and pregnancy outcomes comparable to 8-cell grade I/II embryos, and PCM-8 cell embryos can serve as a valid alternative when good-quality embryos are scarce. Stratification of PCM embryos by cell number facilitates refined clinical decision-making and improves embryo utilization. </p>