Our cycle is so much more than just a week of bleeding each month, that most of us, often dread. It is complex interplay of hormones from our brains and ovaries that operate in a cyclical fashion, which ultimately prepare our bodies for reproduction. Oftentimes the cycle is not really an area women focus on until they are at a time in their lives they are thinking about conception, or if they’ve had health issues directly related to it.
Understanding the cycle, the phases, hormones and what’s happening to your body at various times throughout is incredibly empowering as a female, whether you are trying to conceive or not.
The cycle is typically broken down into two phases. The follicular or proliferative phase, and luteal or secretory phase. Some literature refers to the cycle in four phases adding smaller phases during ovulation and the bleed. Here, we will break down the follicular and luteal phases and ensure to talk about the main event as it relates to fertility– ovulation.
The Follicular Phase
The average length of the female cycle is 28 days although it is still considered normal if it is a few days shorter or longer. It begins on the first day of bleeding. This is when hormones like estrogen and progesterone are lowest because the egg was not fertilized. This fall in hormones triggers not only the shedding of the uterine lining, but the release of another hormone called follicle stimulating hormone (FSH). FSH comes from the brain and stimulates the growth of new follicles inside the ovaries. These follicles produce estrogen, and so early in the cycle estrogen begins to rise again. Estrogen helps to prime and thicken the uterus for implantation. One special follicle, known as the dominant follicle, is recruited and will continue to grow, while the others will not develop any further. As estrogen continues to rise, this stimulates another brain hormone called luteinizing hormone (LH) to rise.
Ovulation
LH continues to rise until it reaches a peak. This peak level of LH is what stimulates the release of the dominant follicle. This is known as ovulation. This occurs about 10-12 hours after peak LH concentration, and 34-36 hours after LH started to rise. Discharge tends to change around this time, to facilitate sperm movement. The time in which there is the highest likelihood of conception is called the fertile window. This contains the day of ovulation, and the 5 days preceding it (the 5 days is due to the length of time that sperm can survive). Although the average female cycle is around 28 days, with ovulation occurring around day 14, this can vary from woman to woman, so it’s important to understand your cycle to determine when your ovulation occurs. Some women use ovulation predictor kits, which involve urine strips that test for the LH surge. Temperature also rises around this time, and some women will track their temperature – this is known as BBT tracking. It’s important as a female to track your cycle, to determine when you ovulate, that way when you are ready to start trying to conceive you can time your intercourse appropriately.
Luteal Phase
This phase starts after ovulation, and most commonly lasts 14 days. LH begins to drop and so does estrogen. The remaining cells within the ovary become a corpus luteum. This secretes mainly progesterone – which is the dominant hormone in the second half of the cycle. Progesterone continues to prepare the uterine lining for implantation. If the egg is fertilized by a sperm, implantation may follow (which is usually 8-9 days after ovulation). The human chorionic gonadotropin (hCG) produced by the developing ovum will continue to drive progesterone to foster a healthy environment for the embryo. If there is no fertilization, or embryo to keep hormones high, estrogen and progesterone will fall, triggering the shedding of the uterine lining. This then begins another cycle.
References:
https://www.nature.com/articles/s41746-019-0152-7
https://www.ncbi.nlm.nih.gov/books/NBK500020/
https://www.ncbi.nlm.nih.gov/books/NBK279054/
