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PURPOSE: To study the morphometric and morphokinetic profiles of pronuclei (PN) between male and female human zygotes. METHOD(S): This retrospective cohort study included 94 consecutive autologous single day 5 transfer cycles leading to a singleton live birth. All oocytes were placed in the EmbryoScope + incubator post-sperm injection with all annotations performed retrospectively by one embryologist (L-SO). Timing parameters included 2nd polar body extrusion (tPB2), sperm-originated PN (tSPNa) or oocyte-originated PN (tOPNa) appearance, and PN fading (tPNF). Morphometrics were evaluated at 8 (stage 1), 4 (stage 2), and 0 h before PNF (stage 3), measuring PN area (um(2)), PN juxtaposition, and nucleolar precursor bodies (NPB) arrangement. RESULTS: Male zygotes had longer time intervals of tPB2_tSPNa than female zygotes (4.8 ± 0.2 vs 4.2 ± 0.1 h, OR = 1.442, 95% CI 1.009–2.061, p = 0.044). SPN increased in size from stage 1 through 2 to 3 (435.3 ± 7.2, 506.7 ± 8.0, and 556.3 ± 8.9 um(2), p = 0.000) and OPN did similarly (399.0 ± 6.1, 464.3 ± 6.7, and 513.8 ± 6.5 um(2), p = 0.000), with SPN being significantly larger than OPN at each stage (p

Background: Recent time-lapse studies have investigated potential live birth prediction by pronuclei (PN) associated morphometric and morphokinetic measures. Whether there is gender associated difference in such measures is unknown. Aim: To determine if morphometric and morphokinetic profiles of PN vary between male and female zygotes. Method: This retrospective study included 94 consecutive autologous single day 5 transfers leading to a singleton live birth (43 males and 51 females) at Monash IVF Gold Coast between January 2019 and March 2020. Timing parameters included 2nd polar body extrusion (tPB2), sperm-originated PN (tSPNa) or oocyte-originated PN (tOPNa) appearance (differentiated by proximity to PB2) and PN fading (tPNF). Morphometrics were evaluated at 8 (stage 1), 4 (stage 2) and 0 hour before PNF (stage 3), measuring PN area (um2), PN juxtaposition, and nucleolar precursor bodies (NPB) arrangement. Results: Male zygotes had longer time intervals of tPB2_tSPNa than female (4.8±0.2 vs 4.2±0.1 h, p=0.044), but not tPB2_tOPNa and tPB2_tPNF. SPN increased in size from stage 1 through 2 to 3 (435.3±7.2, 506.7±8.0, and 556.3±8.9 um2, p=0.000) and OPN did similarly (399.0±6.1, 464.3±6.7, and 513.8±6.5 um2, p=0.000), with SPN being significantly larger than OPN at each stage (p[Formula: see text]0.05 respectively). However, their relative size difference (SPN-OPN) was similar between male and female zygotes at all 3 stages. More male than female zygotes reached central PN juxtaposition at stage 1 (76.7% vs 51.0%, p=0.010), stage 2 (97.7% vs 86.3%, p=0.048) and stage 3 (97.7% vs 86.3%, p=0.048). Furthermore, more OPN showed aligned NPBs than in SPN at stage 1 (44.7% vs 28.7%, p=0.023). No gender associated differences were detected in NPB alignment in either SPN or OPN (p[Formula: see text]0.05 for each comparison). Conclusion: Zygotes with different genders display different morphokinetic and morphometric features. Embryo selection using such parameters may lead to unbalanced sex ratio in the resulting offspring.