Oocyte cryopreservation: when it is done and how it works

The Egg Freezing procedure is indicated in cases where a woman wants to preserve her chance of procreation by postponing it from the time her oocytes are frozen, for various reasons, for example:

• need to undergo therapies that could cause fertility or infertility problems (chemotherapy, radiation therapy, etc.)
• medical conditions that adversely affect fertility (e.g., risk of early menopause, endometriosis, genetic or chromosomal diseases);
• willingness to postpone pregnancy for personal reasons while maintaining egg quality at the time of freezing (so-called social freezing). In fact, as is well known, there is a decline in spontaneous conceptions with increasing age.

The process of oocyte cryopreservation is divided into several steps. Let's look at them below.

First step: preliminary visit

First, a visit for fertility preservation is necessary. At the Natality Science Center of the Gynecology and Obstetrics Unit at San Raffaele, there are 2 outpatient clinics for the management and framing of patients:

• Onco-fertility outpatient clinic: run by a multidisciplinary team of oncology specialists, gynecologists, psychologists, biologists, and obstetricians, accessed by women newly diagnosed with oncological disease who need to undergo treatments that may reduce their fertility;
• Fertility preservation outpatient clinic: dedicated to all patients with benign gynecological and non-gynecological conditions that can reduce their reproductive potential, and patients who choose to invest in this treatment to protect their future fertility.

Second step: pre-operative examinations

An appointment is scheduled at the hospital about a couple of weeks before the start of treatment to perform the preparatory examinations for the procedure, which is aimed at: 

• anesthesiological evaluation;
• performance of blood tests and electrocardiogram;
• counseling with midwife for delivery of examinations and informed consents;
• clarification of the last doubts;
• choice of treatment start date

Third step: treatment

The treatment includes 4 stages:

1. hormone stimulation cycle and ultrasound monitoring;
2. oocyte retrieval;
3. oocyte vitrification in the laboratory;
4. oocyte thawing.

Let's look at them in detail.

Hormone stimulation cycle and ultrasound monitoring

Controlled ovulation induction involves the administration of drugs that act at the ovarian level, stimulating follicle production.

In In-Vitro Fertilization techniques, the drugs used are essentially Gonadotropins, the administration of which is subcutaneous (by injections), daily, for about 2 weeks. The dose of gonadotropins used is determined on the basis of individualized schedules in relation to the patient's clinical characteristics, finalizing the stimulation to produce a higher number of follicles. 

Follicular development is then monitored by repeated ultrasound checks and possibly blood assays of hormone levels until the largest follicles have reached an average diameter around 17-18 mm. In this case, ovulation induction will be scheduled, and 34-36 hours later, the patient will be ready for egg retrieval.

Oocyte retrieval

Oocyte retrieval (oocyte pick-up) is the second stage of the Medically Assisted Procreation procedure, performed by ultrasound-guided transvaginal route under deep sedation. 

In general, the procedure lasts about 10 to 15 minutes and requires hospitalization under Day Surgery. Sampling is performed by means of a special needle mounted on the transvaginal ultrasound probe by which the ovaries are reached and the contents of the follicles are aspirated to collect the oocytes contained therein. 

The aspirated follicular fluid is immediately taken to the laboratory and examined by the biologist for the presence of oocytes.

Oocyte vitrification in the laboratory

The retrieved oocytes are kept for about 2 hours in an incubator. They are then prepared for cryopreservation. 

At this stage, oocytes at an appropriate stage of maturation are identified and cryopreserved (frozen) by vitrification technique and placed in liquid nitrogen where they can be kept for several years.

Oocytes collected at the San Raffaele Center are stored in the cryo-bank located within the Center itself.

Oocyte thawing

When the patient desires a pregnancy, she can use her own cryopreserved oocytes according to current legislation (Law 40/2004). In this case, the oocytes are then subjected to thawing and then to in vitro fertilization, with the partner's sperm, using a Medically Assisted Procreation (MAP) technique, sperm microinjection (ICSI).

It is very important to know that the likelihood of obtaining a good oocyte recovery, and thus a good proportion of oocytes suitable for cryopreservation, is mainly related to the woman's age and her ovarian reserve, which are key factors in determining oocyte quality and response to multiple ovulation induction therapy. 

Indeed, literature data state that after the age of 35 there is a sudden decline in the quality of female gametes and indicate that cryopreservation of at least 8-15 mature oocytes is adequate to ensure a good reproductive prognosis.
In such a situation, the probability of succeeding in pregnancy is estimated at about 40-50%, for women younger than 35 years of age, and then declines dramatically for older women, even with the same number of retrieved oocytes. 

It should be pointed out, however, that in addition to those reported, other elements contribute to success in terms of conception, including: 

• characteristics of the partner's semen;
• condition of the female uterine viscera.

The main risks of oocyte cryopreservation are generally minimal, and are related to:

• ovarian stimulation, at the time when hormone therapy is administered;
• anesthesia;
• surgical harvesting of egg cells. 

The specialist and the Center team properly inform each couple of these eventualities in relation to their medical history before starting the procedure.