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Sunday, 13 April 2014

Ovarian stimulation in IVF: principles

The concepts of ovarian stimulation are not too complicated. First, let us understand what happens in the when no medications are taken.

In the natural cycle, antral follicles start growing (at the behest of FSH from brain) in the initial part of the menstrual cycle. One of these establishes dominance and only this dominant follicle continues to grow. Reason is when follicles start growing, they release estrogen hormone, levels of which starts increasing due to the contribution from the several follicles. As a reaction to the rising estrogen, brain (in particular pituitary) starts decreasing the amount of FSH that is released. With the decreasing levels of FSH, the smaller follicles' growth slows down and eventually stops. Only the dominant follicle has the ability to survive and grow to eventually ovulate and release the egg.

When we stimulate the ovaries, we do not let the FSH levels drop - by giving the same amount of hormone artificially. This allows even the smaller antral follicles to continue their growth. Stimulation is continued till the biggest two-three follicles are ready for ovulation (size ~18mm). 

The other principle that needs to be taken care of is ovulation. In the natural cycle, ovulation is triggered when estrogen hormone rises to a particular level. Ovulation is caused by way of release of a huge amount of hormone LH - this mechanism is called LH surge. As there is only one follicle that is "allowed" to grow, the threshold level of estrogen that triggers LH surge is not very high. 

In the context of ovarian stimulation, multiple follicles are growing and releasing estrogen, as such the level of estrogen required to "trigger" the response is reached quickly. In order to prevent LH surge interfering with the ovarian stimulation, we have to use one of two medications - GnRH analogues (such as Buserelin, Lupride, etc.,) and GnRH antagonists (such as Cetrotide and Ganirelix). These medications are quite different in the way they can be used and I will discuss these in the next post.

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