This first phase of the menstrual cycle begins with the start of bleeding during your period. A hormone called follicle-stimulating hormone (FSH) is released during this phase, which makes the eggs inside the ovaries (follicles) grow. The follicles also produce estrogen, the hormone responsible for maturing your eggs and the uterine lining, as well as moisturizing the cervical fluid critical for pregnancy. This follicular phase lasts around 2 weeks.
When your body reaches an estrogen threshold, it will trigger a surge of luteinizing hormone (LH) which will release the ripest egg, and thus ovulation occurs. Ovulation usually happens between day 11 and day 21 in your cycle.
After the egg is released from the ovary, the follicle that held the egg collapses and becomes an important structure known as the corpus luteum. The corpus luteum produces Progesterone, a hormone that
- Prevents release of other eggs
- Thickens the uterine lining
- Allows a fertilized egg to implant and grow within the uterus
- Changes the three main fertility signs (temperature, cervical fluid, cervical position).
The luteal phase lasts 12 to 16 days in most women, and phases less than 10 days can be problematic for women trying to conceive.
What About Conception?
Women are born with over a million eggs but release only 300-400 eggs in their lifetimes, and usually only one egg per month. When the egg is released from the ovaries, it is picked up by the fallopian tube. If the timing is right, the sperm will fertilize the egg within hours of ovulation. If the egg is not fertilized within 24 hours, then it dies and gets reabsorbed by the body.
In order for fertilization to occur, there are three main players; the egg, the sperm, and cervical fluid. The cervical fluid is produced under the influence of high levels of estrogen in the first phase of the menstrual cycle and directs the sperm through the cervix, a process critical for conception. Because sperm can survive for up to 5 days in this fluid, it is possible to have intercourse on Monday and conceive on Friday!
If the egg is fertilized, it releases human chorionic gonadotropin (HCG), the pregnancy hormone, which signals the corpus luteum to continue releasing progesterone beyond the 16 days in the second half of the menstrual cycle. This is critical to maintaining the uterine lining essential for the newly fertilized egg to survive. After a few months, the placenta takes over maintaining the lining and nourishing the fetus.