Objective <p>To compare embryo development and pregnancy outcomes associated with the use of light versus heavy paraffin oil for embryo culture in patients undergoing in vitro fertilization (IVF).</p> Methods <p>This single-center, prospective, non-randomized cohort study screened 718 IVF cycles between May 2023 and December 2023, of whom 578 were included in the final analysis. Embryological outcomes were assessed for these cycles, frozen embryo transfer and pregnancy outcomes were subsequently followed through January 2025. Embryos were cultured under either light paraffin oil or heavy paraffin oil, with group allocation based on routine laboratory workflow rather than formal randomization. Embryological outcomes included fertilization rate, cleavage rate, proportion of good-quality embryos, blastocyst formation rate, and proportion of good-quality blastocysts. Pregnancy outcomes included positive human chorionic gonadotropin rate, implantation rate, clinical pregnancy rate, ongoing pregnancy rate, live birth rate, and miscarriage rate.</p> Results <p>Among 204 patients in the light paraffin oil group and 374 patients in the heavy paraffin oil group, embryos cultured under heavy paraffin oil showed significantly higher cleavage rate (98.71% vs. 97.57%, <i>p</i> = 0.011) and blastocyst formation rate (63.10% vs. 57.28%, <i>p</i> = 0.007). However, no significant differences were observed in pregnancy outcomes between the two groups.</p> Conclusions <p>Heavy paraffin oil was associated with improved embryo development. However, these laboratory advantages did not translate into improved pregnancy outcomes. Further randomized controlled and multicenter studies are warranted to confirm these findings.</p>