Ovulation isn’t just something that happens randomly; it’s the end of a long chain of complex hormonal events. Think about it like climbing a mountain. You don’t just randomly find yourself at the top of the mountain. You need to hike all the way to the top. Ovulation is just like that.
The graph below shows the hormones that are involved in ovulation, and how they must change throughout the cycle in order for ovulation to occur:
The entire ovulation process is regulated by hormones. During your cycle your body produces five different hormones. The gonadotrophin-releasing hormone (GnRh), follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen and progesterone. They are produced in different parts of your body and regulate what happens during your cycle.1 The process is broken down into three main stages, each of which is distinct due to the ways in which hormones are elevated during that phase.
Let’s take a look at each of the phases of the cycle, and what needs to happen in each phase in order for ovulation to occur:
The first part of your cycle is your follicular phase. It covers the first day of your period all the way until ovulation. At the very beginning of the follicular phase, when you have your period, you are in the process of shedding your uterine lining. The hormones progesterone and estrogen are at their lowest levels.
During the follicular phase, a part of your brain called the hypothalamus is producing gonadotropin-releasing hormone (GnRH). This hormone tells the pituitary gland to start producing follicle stimulating hormone (FSH)2. FSH triggers the growth of multiple follicles inside your ovaries.
These growing follicles secrete increasing amounts of estrogen. Every day of your follicular phase, estrogen levels are rising. Estrogen stimulates the production of fertile cervical mucus and the growth of the uterine lining. Eventually, one of the follicles starts to grow faster than the others. This is called the “dominant” follicle. In a 28-day cycle, the dominant follicle is selected around day 7.
LH Surge and Ovulation
As the dominant follicle continues to grow, it secretes more estrogen. Once estrogen levels are high enough, it stimulates a surge of luteinizing hormone (LH). The LH surge triggers the rupture of the dominant follicle: this is ovulation!
Once the egg is released from the follicle, it makes its way into the fallopian tubes. When the egg is in the fallopian tubes, it is available to be fertilized by sperm for 12 – 24 hours (the best chances of fertilization are when sperm is already waiting in the fallopian tubes by the time the egg is released). If the egg is not fertilized, it is shed along with the uterine lining during menstruation.
After ovulation, LH stimulates the formation of the corpus luteum. The corpus luteum is formed from the remains of the ovarian follicle. The corpus luteum secretes high levels of progesterone and moderate levels of estrogen. The combination of estrogen and progesterone suppresses GnRH, which prevents any more follicles from maturing.
High levels of progesterone during the luteal phase help thicken the uterine lining in preparation for a possible pregnancy. Once LH secretion falls below a certain level, the corpus luteum degenerates, levels of estrogen and progesterone fall, the uterine lining is shed, and the next cycle begins.
- BabyCenter. (2013 April). “How your menstrual cycle works.” Retrieved November 20, 2015, from http://www.babycentre.co.uk/how-your-menstrual-cycle-works ↩
- Klibanski, A. and Tritos, N. ( 2013 May). “Pituitary Disorders.” Hormone Health Network. Retrieved November 20, 2015, from http://www.hormone.org/diseases-and-conditions/pituitary/overview ↩