Total fertilization failure (TFF) is the failure of all metaphase II oocytes to fertilize in ART cycles. The phenomenon represents a known cause of infertility, affecting 1–3% of ICSI cycles. Oocyte activation deficiency (OAD) is the leading cause of fertilization failure, attributed to sperm- or oocyte-related issues, although until recently little attention has been given to oocyte-related deficiencies. Different strategies for overcoming TFF have been proposed in clinical settings, mainly using artificial oocyte activation (AOA) by calcium ionophores. Typically, AOA has been blindly applied with no previous diagnosis testing and, therefore, not considering the origin of the deficiency. The scarcity of data available and the heterogeneous population subjected to AOA make it challenging to draw firm conclusions about the efficacy and safety of AOA treatments. TFF leads to an unexpected, premature termination of ART, which inflicts a substantial psychological and financial burden on patients. This review aims to provide a substantial update on: the pathophysiology of fertilization failure, focusing both on sperm- and oocyte-related factors; the relevance of diagnostic testing to determine the cause of OAD; and the effectiveness and safety of AOA treatments to overcome fertilization failure. Relevant studies were identified in the English-language literature using PubMed search terms, including fertilization failure, AOA, phospholipase C zeta (PLCζ), PLCZ1 mutations, oocyte-related factors, wee1-like protein kinase 2 (WEE2) mutations, PAT1 homolog 2 (PATL2) mutations, tubulin beta-8 chain (TUBB8) mutations, and transducin-like enhancer protein 6 (TLE6) mutations. All relevant publications until November 2022 were critically evaluated and discussed. Fertilization failure after ART has been predominantly associated with PLCζ deficiencies in sperm. The reason relates to the well-established inability of defective PLCζ to trigger the characteristic pattern of intracellular Ca²⁺ oscillations responsible for activating specific molecular pathways in the oocyte that lead to meiosis resumption and completion. However, oocyte deficiencies have recently emerged to play critical roles in fertilization failure. Specifically, mutations have been identified in genes such as WEE2, PATL2, TUBB8, and TLE6. Such mutations translate into altered protein synthesis that results in defective transduction of the physiological Ca²⁺ signal needed for maturation-promoting factor (MPF) inactivation, which is indispensable for oocyte activation. The effectiveness of AOA treatments is closely related to identifying the causal factor of fertilization failure. Various diagnostic tests have been developed to determine the cause of OAD, including heterologous and homologous tests, particle image velocimetry, immunostaining, and genetic tests. On this basis, it has been shown that conventional AOA strategies, based on inducing the calcium oscillations, are highly effective in overcoming fertilization failure caused by PLCζ-sperm deficiencies. In contrast, oocyte-related deficiencies might be successfully managed using alternative AOA promoters that induce MPF inactivation and meiosis resumption. Such agents include cycloheximide, N,N,N′,N′-tetrakis(2-pyridylmethyl)ethane-1,2-diamine (TPEN), roscovitine, and WEE2 complementary RNA. In addition, when OAD is caused by oocyte dysmaturity, applying a modified ovarian stimulation protocol and trigger could improve fertilization. AOA treatments represent a promising therapy to overcome fertilization failure caused by sperm- and oocyte-related factors. Diagnosing the cause of fertilization failure will be essential to improve the effectiveness and safe utilization of AOA treatments. Even though most data have not shown adverse effects of AOA on pre- and post-implantation embryo development, the literature is scarce on the matter concerned and recent studies, mainly using mice, suggest that AOA might cause epigenetic alterations in the resulting embryos and offspring. Until more robust data are available, and despite the encouraging results obtained, AOA should be applied clinically judiciously and only after appropriate patient counseling. Currently, AOA should be considered an innovative treatment, not an established one. Management of fertilization failure includes determining whether it is sperm or oocyte related; artificial oocyte activation and meiosis resumption agents emerge as leading therapies.

The implementation of next generation sequencing (NGS) in preimplantation genetic testing for aneuploidy (PGT-A) has led to a higher prevalence of mosaic diagnosis within the trophectoderm (TE) sample. Regardless, mosaicism could potentially increase the rate of live-born children with chromosomic syndromes, though available data from the transfer of embryos with putative PGT-A mosaicism are scarce but reassuring. Even with lower implantation and higher miscarriage rates, mosaic embryos can develop into healthy live births. Therefore, this urges an explanation for the disappearance of aneuploid cells throughout development, to provide guidance in the management of mosaicism in clinical practice. Technical overestimation of mosaicism, together with some sort of “self-correction” mechanisms during the early post-implantation stages, emerged as potential explanations. Unlike the animal model, in which the elimination of genetically abnormal cells from the future fetal lineage has been demonstrated, in human embryos this capability remains unverified even though the germ layer displays an aneuploidy-induced cell death lineage preference with higher rates of apoptosis in the inner cell mass (ICM) than in the TE cells. Moreover, the reported differential dynamics of cell proliferation and apoptosis between euploid, mosaic, and aneuploid embryos, together with pro-apoptosis gene products (cfDNA and mRNA) and extracellular vesicles identified in the blastocoel fluid, may support the hypothesis of apoptosis as a mechanism to purge the preimplantation embryo of aneuploid cells. Alternative hypotheses, like correction of aneuploidy by extrusion of a trisomy chromosome or by monosomic chromosome duplication, are even, though they represent an extremely rare phenomenon. On the other hand, the technical limitations of PGT-A analysis may lead to inaccuracy in embryo diagnoses, identifying as “mosaic” those embryos that are uniformly euploid or aneuploid. NGS assumption of “intermediate copy number profiles” as evidence of a mixture of euploid and aneuploid cells in a single biopsy has been reported to be poorly predictive in cases of mosaicism diagnosis. Additionally, the concordance found between the TE and the ICM in cases of TE biopsies displaying mosaicism is lower than expected, and it correlates differently depending on the type (whole chromosome versus segmental) and the level of mosaicism reported. Thus, in cases of low-/medium-level mosaicism (<50%), aneuploid cells would rarely involve the ICM and other regions. However, in high-level mosaics (≥50%), abnormal cells in the ICM should display higher prevalence, revealing more uniform aneuploidy in most embryos, representing a technical variation in the uniform aneuploidy range, and therefore might impair the live birth rate.