Ovulation induction

Ovulation induction is the stimulation of ovulation by medication. It is usually used in the sense of stimulation of the development of ovarian follicles to reverse anovulation or oligoovulation, but can also be used in the sense of triggering oocyte release from relatively mature ovarian follicles. In any case, ovarian stimulation (in the sense of stimulating the development of oocytes) is often used in conjunction with ("strict") ovulation induction. Also, a few definitions also include ovarian hyperstimulation (stimulating the development of multiple follicles of the ovaries in one single cycle) in the definition of ovarian stimulation. Otherwise, ovarian hyperstimulation may still be a side effect of ovulation induction.

Reversing anovulation or oligoovulation
Ovulation induction in the sense of reversing anovulation or oligoovulation is indicated for women who do not ovulate on their own regularly, such as those with Polycystic ovary syndrome (PCOS). The medication which is most commonly used to treat anovulation is clomifene citrate (or clomid), which is a selective estrogen receptor modulator (SERM) that increases production of gonadotropins by inhibiting negative feedback from estrogen on the hypothalamus.

Also, where anovulation or oligovulation is secondary to another disease, the treatment for the underlying disease can be regarded as ovulation induction, by indirectly resulting in ovulation.

Triggering oocyte release
Ovulation induction in the sense of triggering the release of oocytes from already relatively developed follicles can be more more specifically termed oocyte release triggering. Colloquially, this is known as the "trigger shot." However, the process generally includes acceleration of the final maturation of oocytes as well, and thereby partly overlaps with the processes of ovarian hyperstimulation and reversal of anovulation or oligovulation. The triggering of oocyte release is equivalent to the physiological role the LH surge by luteinizing hormone.

Administration of medication for triggering of oocyte release avails for sexual intercourse or intrauterine insemination to be scheduled at ovulation, the most likely time to achieve pregnancy. In in vitro fertilization, it avails for final maturation of the oocytes and proper timing of egg retrieval.

Medications that can trigger oocyte release include:
 * A low dose of human chorionic gonadotropin (HCG or hCG), which may be injected after completed ovarian stimulation. Ovulation will occur between 38 and 40 hours after a single HCG injection. It is also used in in vitro fertilization, where it makes the follicles perform their final maturation. A transvaginal oocyte retrieval is then performed at a time usually between 34 and 36 hours after hCG injection, that is, just prior to when the follicles would rupture. This avails for scheduling the egg retrieval procedure at a time where the eggs are fully mature. HCG injection confers a risk of ovarian hyperstimulation syndrome.
 * Oocyte release triggering with a GnRH agonist is a valid alternative to HCG triggering, resulting in an elimination of the risk of ovarian hyperstimulation syndrome. The delivery rate is approximately 6% less than with hCG triggering.

Alternatives
In vitro maturation is letting ovarian follicles mature in vitro, and this technique can potentially be an alternative both to anovulation reversal and oocyte release triggering. Rather, oocytes can mature outside the body, such as prior to IVF. Hence, no (or at least a lower dose of) gonadotropins have to be injected in the body. However, there still isn't enough evidence to prove the effectiveness and security of the technique.