Hormones control the human body’s reproductive system. The two hormones secreted by the pituitary glands are follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both of which are secreted in response to the release of GnRH from the hypothalamus. FSH travels to the ovaries and stimulates a group of follicles to grow. These primordial follicles develop into primary follicles and then secondary follicles.
Follicle stimulating hormone (FSH) is one of the gonadotrophic hormones. FSH is one of the hormones essential to pubertal development and the function of women’s ovaries and men’s testes. In women, this hormone stimulates the growth of ovarian follicles in the ovary before the release of an egg from one follicle at ovulation.
Luteinizing hormone (LH) is produced and released in the anterior pituitary gland. It controls the function of ovaries in females. Two weeks into a woman’s cycle, a surge in luteinizing hormone causes the ovaries to release an egg during ovulation. If fertilization occurs, luteinizing hormone will stimulate the corpus luteum, which produces progesterone to sustain the pregnancy.
During the time of puberty in females, out of 60,000 to 80,000 available primary oocytes, only 400 of them gets a chance to form a mature oocyte. Only one of the mature oocyte out of the 400 will be released every month which can get fertilized when it comes in contact with the sperm. 
How does all this happen?
The hypothalamus produces a hormone called the gonadotropin-releasing hormone or GnRH. GnRH stimulates the anterior lobe of the pituitary to secrete follicle-stimulating hormone (FSH).FSH travels to the ovaries and stimulates a group of follicles to grow, were one of them will survive and become a mature follicle, the rest die.
FSH stimulates estrogen production. The increasing level of estrogen acts on the hypothalamus and the anterior pituitary to increase the level of GnRH and induce the production of another hormone, luteinizing hormone (LH). 
A surge in LH secretion triggers ovulation – the release of the egg from the follicle and the ovary.
Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest section of the Fallopian tube. The fertilized egg immediately begins the process of development while travelling toward the uterus.
Textbook of medical physiology, 11th edition , Arthur C. Guyton and John E. Hall.
Naturally, in each ovarian cycle one follicle becomes mature and gets ready for fertilization. Now women who have infertility problem and is going to undergo IVF, FSH are given as subcutaneous (under the skin) injection that will regulate ovulation, the growth and development of eggs in the ovaries.
After FSH treatment, ultrasound scans are needed to monitor the response of the follicles growth in the ovaries. The growth of the follicles is assessed by observing their increase in size using a trans-vaginal ultrasound.
If the follicles seen on the scan is in the range of 16 mm to 20 mm in size, then the trigger shot is given as mentioned in the next step. This hormone initiates the final maturation and release of the eggs. This mimics the LH surge that stimulates ovulation during normal cycle.
Taking a hCG shot:
The next step in IVF treatment is triggering the oocyte for the last stage of maturation, before retrieval. This last growth is triggered with an injection. This is also called the “hCG (human Chorionic Gonadotrophin) Trigger shot”.
The injection is given when the follicles have grown in range of 16 to 20 mm in size. This shot is typically a one-time injection.
Going for the Gold- Retrieval of Eggs:
About 34 to 36 hours after the “trigger shot” is received, the egg retrieval or ovum pick up will take place.