During follicular puncture, not all retrieved oocytes present the same degree of maturation. Although most of them are at the optimal stage for fertilization, some have not yet completed this process and remain in a phase known as metaphase I or the MI stage.
Traditionally, these immature oocytes have been considered to have little reproductive value and, in many cases, have been discarded during the laboratory process. However, some of them are capable of completing their maturation spontaneously in the laboratory before intracytoplasmic sperm injection (ICSI), raising an increasingly relevant question in assisted reproduction: is it worth using them?
Three studies presented by IVF-Life professionals at the annual congress of the European Society of Human Reproduction and Embryology (ESHRE) sought to answer this question by analyzing the reproductive potential of these oocytes from different perspectives.
The first study, led by Alba Alarcón, an embryologist at IVF-Life Madrid, analyzed more than 65,000 oocytes obtained from in vitro fertilization treatments carried out between 2019 and 2026, comparing those that had reached maturity naturally within the ovary with others that completed the process later in the laboratory.
The results showed that oocytes maturing in vitro have a lower capacity for fertilization and a significantly lower probability of developing to the blastocyst stage. In other words, from a statistical perspective, these oocytes have lower reproductive potential than those that were already mature at the time of ovarian retrieval.
The second study, presented as an oral communication at the congress by Clara Pérez de Blas, an embryologist at IVF-Life Madrid, and also led by Alba Alarcón, focused on the embryos that do successfully develop from these oocytes.

The analysis showed that blastocysts derived from oocytes that complete their maturation in the laboratory reach this stage in timeframes similar to embryos originating from oocytes that were mature from the beginning of treatment. However, some differences were observed in their morphological characteristics. These embryos more frequently displayed earlier expansion stages and lower-quality trophectoderm, the cellular structure that will later give rise to the placenta.
These findings suggest that, although the embryos retain the ability to progress to advanced stages of development, there are biological differences that should be taken into account during embryo selection and patient counseling.
The third study, once again led by Alba Alarcón, analyzed the rate of euploidy, that is, the proportion of embryos with the correct number of chromosomes, and found that blastocysts obtained from oocytes matured in the laboratory showed results very similar to those derived from oocytes that were already mature at retrieval.
This means that, although these oocytes face greater challenges in becoming viable embryos, those that do manage to overcome the early stages of development maintain comparable chromosomal competence.
The results of these studies make it possible to redesign the strategies that have been used in recent years. Immature oocytes do not provide the same performance as mature oocytes obtained during retrieval and, therefore, cannot be considered equivalent from a reproductive standpoint. However, neither do they appear to be oocytes without clinical utility.
In fact, in certain situations, especially in patients with a poor ovarian response or in cycles where only a limited number of oocytes are retrieved, making use of this small additional reserve could help increase the total number of embryos available and, consequently, the cumulative chances of achieving pregnancy.
Reproductive medicine is moving toward increasingly personalized treatments and, in this context, understanding the true potential of each oocyte makes it possible to make more precise decisions and offer new opportunities to patients who, until relatively recently, had fewer alternatives available.