How does a woman’s age influence the oocytes (eggs) quality?

Oocytes are the key factor in fertility. As the rarest cells of the human body and yet the most rapidly ageing they are decisive in the outcome of IVF treatment. During this webinar Dr Łukasz Sroka from InviMed answers the question how a woman’s age influences oocyte quality.

The human genetic material in oocytes

Oocytes, just like sperm, contain only half of human genetic material. While sperm is produced every day and almost all the time, oocytes, are formed very early on in the human development. During the meiotic maturation division process, through which immature oocytes become eggs, oocytes go into the so-called primary arrest. In other words, they are stopped during the division for a very long time, even decades. This special feature of oocytes is very important when we compare oocyte quality versus a woman’s age.

Oocytes quantity

Dr Sroka starts with describing Oocyte quantity. He bases his explanation on the well-known publication by Wallace and Kelsey who analysed a lot of human tissue from different sources as well as what happens to oocytes during the whole of a woman’s life – from seven weeks after conception up to 51 years old. According to it, NGF (non-growing follicle) is the base for ovarian reserve, the number of oocytes in the ovaries. Human ovaries do not produce eggs every day all the time. The production happens when a woman is still in her mother’s womb and then the number is constant. It just decreases with the age.

Dr Sroka says that the number of oocytes in the ovaries is fixed on the day of birth. The average number we come across most often is about 300,000-400,000 of NGF on the day of a woman’s birth. This is very important information because it means that a woman can be born with different number of non-growing follicles and this number will decrease with age.

We cannot assume that the speed at which the number of oocytes is decreasing is similar across the female population. Usually we think that a woman aged 25 is quite capable of being pregnant and at this time the average number of remaining non-growing follicle population is about 20%. Nowadays, the most typical age for getting pregnant in Western countries is about 30 years plus. And currently there is only 10-12 % of NGF population. On the average, the non-growing follicle population decreases up to the age of 50 when their number is exceptionally low. Of course there is also a small group of women for whom this number starts from much lower values and decreases much faster. They can achieve a kind of premature menopause at 40 or below. Conversely, some women with ‘hyper fertility’ can get pregnant at a much older age.

Dr Sroka makes another significant point here. There is no way that IVF stimulation will deplete a woman’s own ovarian reserve faster than it happens normally. A woman loses her eggs every month at an exceedingly high speed. The maximum is about 900 follicles per month on average. This number of course decreases also with the age. What is interesting is that probably the more NGF you have, the faster you lose them.

Oocytes quality

When it comes to oocyte quality, there is this already mentioned moment of arrest when the oocyte is kept in the middle of division. The doctor recalls another publication where the authors examined a very large number of oocytes acquired from women at different age and checked the normality of chromosomes in these oocytes. It was found out that as a woman’s maternal age increases so do the number of aneuploid (impaired) oocytes. At the age of 45 it is difficult to find any healthy oocytes. You might still have a plenty of oocytes in your ovaries but their quality is rather poor.

The relation between oocytes quantity and quality

There are a few clinical implications related to the quantity and quality of oocytes. Dr Sroka presents the information from the Centers for Disease Control and Prevention (CDC). They collect data from almost all IVF clinics in the United States. There is a sharp decline in pregnancy rate and live birth after the age of 36 or 37. Around this time, human oocytes become much worse in quality. That is why most IVF clinics in the world do not perform IVF on women after 43/44 years old. The pregnancy rate and the live birth rate for women at 40-42 is about 10-12%. We must bear in mind that the quality of oocytes affects the results of IVF procedure. Additionally, after 40 there is a sharp increase in the miscarriage rate.

However, even at the age of 44-45 it is possible to achieve 75% of positive results of oocytes pick-up procedure. But despite that the pregnancy rate is as low as 4% and the live birth rate is about 3% (including babies with genetic disorders). Additionally, embryos received from unhealthy oocytes will not implant. Of course, in case of egg donors, we get a constant number of pregnancies regardless of woman age. This all points to the fact that a woman’s fertility is related to the age of her oocytes. That is the reason women have to think about getting pregnant faster.