Association between sperm morphology and embryo cleavage rate in in vitro fertilization: a retrospective study

Introduction / Введение

Infertility is a reproductive health issue affecting approximately 8–12 % of couples worldwide. One of the commonly employed methods to address infertility is assisted reproductive technology, such as in vitro fertilization (IVF) [1]. Several indicators can determine the success of IVF, including sperm motility, blastocyst development rate, fertilization rate, embryo development rate, implantation rate, and cleavage rate [2]. According to the European Society of Human Reproduction and Embryology (ESHRE), cleavage rate is an important indicator because it reflects how well the culture system supports cell division, assesses the embryo’s growth potential, and detects contamination within the culture [2]. One factor that can influence cleavage rate is sperm quality, particularly sperm morphology [3].

Normal sperm morphology indicates a well-structured sperm suitable for fertilization, whereas abnormal sperm morphology is often associated with impairments in fertilization and embryo development processes [4]. Good sperm morphology is linked to a reduced risk of genetic material damage, as sperm genetic integrity plays a crucial role in embryo cleavage [5]. Several studies have demonstrated that sperm with abnormal morphology tend to have difficulties penetrating the oocyte, which can adversely affect cell division and the quality of resulting embryos [6][7]. Poor sperm morphology is associated with DNA damage that leads to chromosomal misdistribution, resulting in aneuploidy and cleavage arrest at the 8-cell stage [8]. Additionally, sperm with abnormal morphology exhibit reactive oxygen species (ROS) levels 2.5 times higher, which can reduce blastocyst formation by 34 % [9].

Sperm quality can be influenced by several factors, including motility, sperm count, as well as external factors such as lifestyle and medical conditions of the male partner [4]. In IVF procedures, the selection of the best sperm is performed using specific techniques such as swim-up or density gradient centrifugation to enhance the likelihood of successful fertilization [10]. A previous study conducted in Cleveland in 2011 found no significant difference in cleavage rates between patients with poor sperm morphology (98 %) and those with normal sperm morphology (99 %) undergoing intracytoplasmic sperm injection (ICSI) [3]. Conversely, a study conducted in India in 2023 reported different findings, where subjects with normal sperm morphology had a higher average cleavage rate (68 %) compared to those with abnormal morphology (54.4 %) [11]. Therefore, it remains a subject of debate whether selecting sperm with optimal morphology during IVF procedures significantly improves cleavage rates.

Aim: to analyze the association between sperm morphology and embryo cleavage rate in patients undergoing IVF/ICSI.

Materials and Methods / Материалы и методы

Research tasks / Задачи исследования

The research tasks of this study are as follows:

1. to describe the distribution of cleavage rates (rapid, normal, and slow) among embryos produced from IVF/ICSI procedures at Dr. Hasan Sadikin Central General Hospital;
2. to analyze the statistical association between sperm morphology categories and cleavage rate groups using the Chi-Square test;
3. to determine the odds ratio (OR) of experiencing rapid or slow cleavage in patients with teratozoospermia compared to those with normospermia, with a 95 % confidence interval (CI).

Study design / Дизайн исследования

This study is an analytical retrospective study. The research was conducted to analyze the relationship between sperm morphology and cleavage rate in infertile couples undergoing IVF programs at Dr. Hasan Sadikin Central General Hospital. Data collection was performed using medical records obtained from the period of January 2020 to December 2024.

The study population consisted of all infertile couples undergoing IVF treatment using ICSI method at Dr. Hasan Sadikin Central General Hospital who met the inclusion and exclusion criteria. Data were collected using a total sampling method, followed by selection based on inclusion and exclusion criteria to obtain a sample with the desired characteristics.

Inclusion and exclusion criteria / Критерии включения и исключения

Inclusion criteria: infertile couples undergoing IVF with the ICSI method, availability of complete sperm analysis data including morphology parameters based on the manual laboratory examination according to WHO 2021 guidelines, and availability of embryo development data up to day 2 post-fertilization (cleavage rate).

Exclusion criteria: patients with medical conditions that could affect IVF outcomes, such as genetic disorders or chromosomal abnormalities.

Study methods / Методы исследования

Samples were selected using total sampling according to inclusion criteria: complete semen analysis including sperm morphology based on WHO 2021 criteria and available embryo development data to assess cleavage rate; patients with genetic or chromosomal disorders will be excluded. Sperm morphology, categorized as normal or abnormal, is the independent variable, while cleavage rate, categorized as rapid, normal, or slow according to the predefined laboratory criteria, is the dependent variable.

Statistical analysis / Статистический анализ

Data were analyzed using SPSS version 26.0. The normality of numerical variables was assessed using the Kolmogorov–Smirnov test. Non-normally distributed numerical variables were presented as median and range or interquartile range, while categorical variables were presented as frequencies and percentages. The association between sperm morphology and cleavage-rate categories was analyzed using the Chi-square test or Fisher’s exact test, as appropriate. Odds ratios (ORs) with 95 % confidence intervals (CIs) were calculated for rapid versus normal cleavage and slow vs. normal cleavage. A p-value < 0.05 was considered statistically significant.

Data confidentiality / Конфиденциальность данных

To ensure data confidentiality, all patient data were anonymized. Access to the dataset was restricted to the research team, and electronic data were deleted from the researchers’ repository within five years after the completion of the study.

Results / Результаты

A total of 159 records were identified and included in the analysis. The participants’ characteristics are presented in Table 1. The mean age for the female patients is 33.56 ± 4.15 years and the mean age for male patients is 36.29 ± 5.16 years. The mean for body weight is 60.05 ± 6.301 kg, the mean height is 1.55 ± 0.047 m, and the mean body mass index is 24.97 ± 3.235.

The comparison of association between sperm morphology and cleavage rate is shown in Table 2. In the rapid cleavage group, patients with normal sperm morphology accounted for 1 case (3.0 %), while those with teratozoospermia comprised 32 cases (97.0 %). In the normal clea­vage group, patients with normospermia numbered 49 cases (52.7 %), and those with teratozoospermia totaled 44 cases (47.3 %). In the slow cleavage group, patients with normospermia were 2 cases (6.1 %), whereas teratozoospermia cases amounted to 31 (93.9 %).

For categorical data analysis in the above table, the Chi-square statistical test was applied to evaluate sperm morphology. The results revealed a statistically significant p < 0.001 for the sperm morphology variable. This indicates a significant difference in proportions between sperm morphology categories across the cleavage rate groups. Thus, it can be concluded that sperm morphology distribution varies significantly among the cleavage rate cohorts.

In the rapid cleavage group, patients with sperm morphology categorized as teratozoospermia accounted for 32 cases or 97 %, while those with normospermia accounted for only 1 case or 3 %. In the normal cleavage group, patients with teratozoospermia were 44 cases or 47.3 %, and those with normospermia were 49 cases or 52.7 %. To analyze the categorical data in the table above, a Chi-Square statistical test was employed to assess sperm morphology. The statistical analysis revealed that the p-value for the sperm morphology variable was less than 0.001 (p < 0.001), indicating statistical significance. Therefore, it can be concluded that there is a statistically significant difference in the proportion of sperm morphology between the rapid cleavage and normal cleavage groups (Table 3). Patients with teratozoospermia had 35.64 times the odds of rapid cleavage compared with those with normospermia (OR = 35.636; 95 % CI = 4.673–271.760; p < 0.001).

In the slow cleavage group, patients with sperm morphology categorized as teratozoospermia accounted for 31 cases or 93.9 %, while those with normospermia accounted for 2 cases or 6.1 %. In the normal cleavage group, patients with teratozoospermia were 44 cases or 47.3 %, and those with normospermia were 49 cases or 52.7 %. To analyze the categorical data presented in the table above, a Chi-Square statistical test was employed to assess the variable of sperm morphology. The statistical analysis indicated that the p-value for the sperm morphology variable was less than 0.001 (p < 0.001), signifying statistical significance. Accordingly, it can be concluded that there is a statistically significant difference in the proportion of sperm morphology between the slow cleavage and normal cleavage groups (Table 4). Patients with teratozoospermia had 17.26 times the odds of slow cleavage compared with those with normospermia (OR = 17.261; 95 % CI = 3.903–76.340; p < 0.001).

Discussion / Обсуждение

The findings of this study demonstrate a statistically significant association between sperm morphology and cleavage rate in patients undergoing IVF, as evidenced by the Chi-square test results (p < 0.001). The predominance of teratozoospermia in both the rapid cleavage rate (97.0 %) and slow cleavage rate (93.9 %) groups, compared to the more balanced distribution in the normal cleavage rate group (52.7 % normospermia vs. 47.3 % teratozoospermia), suggests that abnormal sperm morphology may compromise the timing and quality of embryo division.

The results from this study partially contradict earlier studies, such as the 2011 Cleveland trial reporting no significant cleavage rate differences between normal and abnormal sperm morphology groups. That study analyzed 235 women and found no difference in fertilization rate and pregnancy rate in group A (0–2 % normal morphology) and group B (5–13 % normal morphology). The authors also found that significantly fewer embryos from group A had undergone compaction compared to group B embryos (p = 0.0007). But in Day 5, group A blastulation was 34.0 % compared to group B 38.0 % blastulation (p > 0.05). This study concluded that blastulation rates were not significantly different even though compaction rates showed a delay in group A [3]. In contrast, this study resonates with findings from the 2023 Indian study, which observed superior cleavage rates in normospermic samples. In this study, it was found that the cleavage rate was higher in subjects with normal sperm morphology at 3.0 % (67.94) compared to 2.0 % (60.87) and 1.0 % (54.38). The study also employed the Cramér’s V test to assess the strength of the correlation between sperm morphology and cleavage rate. A Cramér’s V value of 0.714 was obtained, indicating a strong association between sperm morphology and cleavage rate. Furthermore, this correlation was reinforced by the finding that a higher proportion of normal sperm morphology was associated with an increased average cleavage rate [11].

Several factors may explain why this phenomenon occurs. The first is the morphology of the acrosome structure. The acrosome plays a crucial role in the structural changes that occur when sperm encounters the oocyte. These processes include capacitation, the acrosome reaction, and the release of enzymes. If the acrosome is improperly formed, these processes cannot proceed effectively, thereby preventing fertilization. The shape of the sperm head is also important for recognition and binding to the zona pellucida of the oocyte. Sperm with abnormal head morphology are less likely to be recognized by the oocyte, hindering the fusion between the acrosome and the zona pellucida [12][13]. Poor sperm morphology may also reflect DNA fragmentation within the sperm nucleus. This occurs due to failure in the repair of nuclear DNA during the spermatid stage. Sperm exhibiting large residual cytoplasm are associated with increased DNA strand breaks and elevated ROS production [14]. A study conducted in Bulgaria found that defects in the sperm head are associated with late-stage embryo development, defects in the neck (midpiece) correlate with intermediate embryo cleavage, and tail defects are related to early stages of embryo development. Additionally, sperm with head defects have been linked to delayed blastocyst formation (t9–tHB) [15].

This study’s retrospective design limits causal inference, and confounding variables such as female age, ovarian response, or embryological techniques were not controlled for. Future prospective studies with larger cohorts and standardized protocols are needed to isolate the impact of sperm morphology while accounting for female and procedural factors. Additionally, extending follow-up to later developmental stages (e.g., blastocyst formation or live birth rates) could clarify whether cleavage rate discrepancies translate to clinically meaningful outcomes.

Conclusion / Заключение

There is a statistically significant association between sperm morphology and cleavage rate in patients undergoing IVF treatment. Specifically, the distribution of sperm morphology categories (normospermia vs. teratozoospermia) differs significantly across the rapid, normal, and slow cleavage rate groups. The rapid cleavage group was predominantly composed of patients with teratozoospermia (97.0 %), while the normal cleavage group showed a more balanced distribution and the slow cleavage group was also predominantly composed of patients with teratozoospermia (93.9 %). This suggests that abnormal sperm morphology (teratozoospermia) may be associated with altered timing of early embryo cleavage, including both rapid and slow cleavage patterns.