2. INTRODUCTION
The first IVF baby was born during a natural
(unstimulated) IVF cycle.
However, it was soon recognized that the
success rate of IVF in natural cycles was low,
primarily due to the low number of oocytes
retrieved.
Ovarian stimulation using urinary gonadotropins
was adopted to deal with this problem, resulting
in a significant increase in both the number of
eggs retrieved, as well as the success rate of IVF.
3. With the increasing use of stimulation in IVF
cycles, various problems were recognized.
Premature luteinization and failure of
synchronous follicular recruitment due to early
dominant follicle selection were the two main
problems resulting in reduced success rates.
Also, ovulation could occur at any time of the
day necessitating intensive monitoring and
oocyte retrieval at inconvenient times of the day
4. Gonadotropin-releasing hormone agonists
(GnRHa) were demonstrated to result in
pituitary desensitization and successfully
dealt with these problems, becoming the
next major breakthrough in IVF treatment.
5. More recently gonadotropin releasing
hormone antagonists (GnRHant), which have
a similar function but take a shorter period of
time to achieve pituitary suppression (and
prevent the premature LH surge) are
increasing being used for the same purpose.
6. In the natural cycle, follicular dominance is
achieved by the estradiol induced negative
feedback on the pituitary gland, causing a
decline in FSH below threshold levels.
In IVF cycles, exogenous gonadotropins are
used to achieve supra-threshold levels of
gonadotropins during the phase of follicular
recruitment to interfere with this process of
dominant follicle selection and enable
multiple follicular recruitment
7. Pituitary desensitization by using either
GnRH agonists or GnRH antagonists, is an
important part of IVF stimulation as it
eliminates the possible interference by
endogenous hormones, enabling
synchronous follicular development.
This prevents premature luteinization, and
helps to reduce intensive and frequent
monitoring allowing control over the timing
of oocyte retrieval.
8. The LH surge is substituted by exogenous
hCG, enabling clinics to time oocyte retrieval.
