What do immature oocytes mean? Causes and solutions

The importance of oocyte maturity in the IVF program

One of the key steps in the in vitro fertilization (IVF) process is to obtain mature and high-quality oocytes capable of being fertilized. Normally, the follicular puncture should yield between 75 % and 90 % mature oocytes. These are the ones able to successfully fertilize. If the proportion of immature oocytes does not exceed 10 to 25 %, this significantly reduces the overall effectiveness of the program (see Human Reproduction, 2018; ASRM Practice Committee, 2021; Frontiers in Endocrinology, 2023).

Immature oocytes cannot be fertilized because an important genetic process has not been completed: the reduction of the number of chromosomes and the preparation for fusion with the sperm genes. As a result, these oocytes do not form a correct chromosomal set, making them unsuitable for conception (ESHRE Guidelines, 2020; Fertility and Sterility, 2019; Journal of Assisted Reproduction and Genetics, 2025).

What does oocyte maturity mean?

The process of ovum formation and maturation is very complex and differs from the development of male reproductive cells. The stock of oocytes forms in the girl from the intrauterine period and gradually depletes from birth. Until ovulation, each ovum contains 46 chromosomes and remains immature. Maturation begins with the formation of the dominant follicle and is completed at ovulation.

At the moment of ovulation, the ovum progresses to metaphase II of meiosis and becomes fully prepared for fertilization, containing 23 chromosomes. This ensures a correct genetic set for the future child. During follicular growth stimulation in IVF programs, the doctor’s goal is to obtain mature oocytes for cryopreservation or fertilization. In women, ovulation generally occurs around the 14th or 15th day of the menstrual cycle, and it is at this time that oocytes mature naturally.

A mature oocyte is most often found in a follicle with a diameter of 18 to 22 mm. That’s why the puncture is performed precisely at this moment, when most follicles reach their optimal size. Embryologists evaluate the maturity and quality of oocytes 2 to 4 hours after puncture. If some are immature, the laboratory creates the conditions necessary for their in vitro maturation. However, the likelihood of obtaining chromosomally healthy embryos from these oocytes is significantly lower than from mature MII-stage oocytes.

Why do we obtain immature oocytes?

The reasons can be various:

• insufficient surge of luteinizing hormone (LH), responsible for final maturation;
• inadequate size of follicles at the time of ovulation trigger;
• genetic peculiarities – mutations in LH or FSH receptor genes;
• age-related changes in reproductive function;
• use of certain medications reducing sensitivity to LH.

All these factors cause some oocytes to remain immature on the day of puncture.

What happens if an immature oocyte is fertilized?

Fertilization of immature oocytes is impossible and dangerous. Normally, both the ovum and spermatozoon must transmit 23 chromosomes each. An immature oocyte retains a complete set of 46 chromosomes, which makes the formation of a correct genome in the embryo impossible. This results in the failure to develop a viable embryo.

What should be done if immature oocytes have been obtained during the previous IVF cycle?

In case of a high proportion of immature oocytes, a thorough examination and protocol modification are necessary:

• analysis of follicle size and timing of ovulation trigger administration;
• assessment of the diameter of the main follicles on the trigger day;
• genetic testing to detect anomalies in hormonal receptors;
• selection of the optimal ovulation trigger or use of a double trigger to improve response;
• adjustment of the interval between trigger administration and puncture.

These measures require a high level of expertise from the reproductive gynecologist and can significantly increase the proportion of mature oocytes in subsequent cycles. This leads to higher chances of obtaining healthy embryos and giving birth to a child.

IVF and protocol specifics

• In vitro fertilization (IVF): in the standard protocol, the main goal is to maximize the number of mature oocytes, as they directly influence the results.
• IVF with egg donation: in these programs, donated oocytes are carefully selected based on maturity and quality criteria. Immature cells are excluded to ensure a high fertilization rate.
• Double donation IVF: when both oocytes and sperm from donors are used, the chances of successful conception are even higher, but monitoring oocyte maturity remains a crucial step in preparing the procedure. Thus, immature oocytes are one of the common causes of reduced effectiveness of IVF programs. However, modern diagnostic methods, careful protocol selection, and the use of in vitro oocyte maturation techniques significantly increase treatment success and bring patients closer to their main goal: the birth of a healthy child.

The NatuVitro clinic adheres to the strictest standards and demonstrates great responsibility in oocyte selection, ensuring rigorous quality control and patient safety.

In our blog, we also addressed the topic of “evaluation of oocyte quality in the IVF program,” where this subject is covered in detail.