In women of reproductive age with normal ovarian function (excluding those with premature ovarian failure or ovulation disorders), each menstrual cycle begins with a group of follicles (ranging from a few to several dozen) that compete to grow and mature. However, under natural hormonal conditions, only one follicle receives enough hormonal support to mature, while the others do not get sufficient nourishment and become suppressed.

The dominant follicle also secretes a hormone called inhibin, which further inhibits the growth of other follicles. As a result, these follicles do not develop fully and undergo a process called atresia (follicular degeneration). In a natural cycle, typically only one follicle matures and releases one egg.

If we were to collect eggs naturally, we could usually only retrieve one follicle, with a 50%-80% chance of actually obtaining an egg. Even if an egg is retrieved, it may not be mature, and there is a possibility of fertilization failure, leading to a very low efficiency of egg collection under natural conditions. To increase the chances of pregnancy, ovarian stimulation (controlled ovarian hyperstimulation) is used in IVF.

The Science Behind Ovarian Stimulation:

Doctors understand that a woman’s ovaries contain about 2 million follicles at birth. By puberty, this number drops to around 500,000 follicles. Throughout a woman’s reproductive life, only about 300 follicles will develop into dominant follicles capable of ovulation, while the rest undergo atresia without ever maturing. Importantly, these follicles that undergo atresia are not necessarily of poor quality.

To take advantage of these otherwise lost follicles, doctors use medication to stimulate the ovaries, providing additional hormonal support (like “extra nourishment”). This allows more follicles, which would typically degenerate, to mature and develop into dominant follicles.

Benefits of Ovarian Stimulation:

  • Increases the number of mature eggs retrieved: By stimulating multiple follicles, the chances of obtaining mature eggs are greatly improved, making the process more efficient.
  • Improves fertilization and embryo quality: With more eggs retrieved, there are more opportunities to fertilize and create viable embryos.
  • Increases the chances of a successful IVF cycle: Instead of retrieving just one egg, ovarian stimulation allows for the collection of multiple eggs, which can then be fertilized to create multiple embryos.
  • Enables the possibility of multiple embryo transfers: Having multiple embryos available allows for several transfer attempts, increasing the likelihood of achieving pregnancy and even enabling the option of having more than one child from a single IVF cycle.

The Process of Ovarian Stimulation:

During the stimulation phase, medications are administered to stimulate the ovaries for about 10-12 days. These medications, often given via subcutaneous injections, encourage more follicles to grow. The development of the follicles is monitored regularly with blood tests and ultrasounds to track hormone levels, follicle size, and endometrial thickness.

Key Points to Remember:

  • The follicles that grow during ovarian stimulation would have either matured or undergone atresia in a natural cycle anyway. Ovarian stimulation does not deplete the future egg reserve and does not cause early menopause.
  • By promoting the growth of multiple follicles, doctors can increase the efficiency of egg retrieval, making the IVF process more effective and improving the chances of pregnancy.

In summary, ovarian stimulation is a critical component of the IVF process. It allows doctors to maximize the number of eggs retrieved in a single cycle, increasing the likelihood of successful fertilization, embryo development, and ultimately, pregnancy. This approach helps make the most of each IVF cycle, giving couples the best chance for a successful outcome.