Phases of the Menstrual Cycle and Pregnancy: How the Fertility Cycle Works
Key takeaways:
The menstrual cycle can last anywhere between 21 and 35 days. The average menstrual cycle lasts 28 days.
The menstrual cycle or fertility cycle is broken down into four phases: menstruation, the follicular phase, ovulation, and the luteal phase.
You’re most likely to get pregnant during ovulation. But each phase of the menstrual cycle affects your fertility.
If you’re trying to get pregnant, you’re probably taking a deep dive into how the menstrual cycle works and how it affects your fertility.
Understanding your menstrual cycle can help you figure out when you’re most likely to get pregnant. If you’re using intrauterine insemination (IUI) or in vitro fertilization (IVF) to get pregnant, understanding your menstrual cycle can also help you plan your cycle and know what to expect.
Here’s a look at the phases of the menstrual cycle and how they affect your fertility.
Search and compare options
What is the menstrual cycle?
The menstrual cycle describes the growth and shed of the lining of the uterus. In short, it’s the number of days between the first day of your period and the day before your next period. The average menstrual cycle is about 28 days, or one month. Some people have shorter or longer cycles so that the normal range is anywhere between 21 and 35 days.
People with irregular periods have menstrual cycles that are either shorter or longer than this average range. When it comes to fertility, timing is everything. So if your menstrual cycle is too long or too short, it can affect your chances of getting pregnant.
The menstrual cycle is broken down into four phases: menstruation, the follicular phase, ovulation, and the luteal phase. You’re most fertile during ovulation. But all the phases work together so that you can get and stay pregnant.
Menstruation
The menstrual cycle always starts with menstruation, or your period. Day one of each fertility cycle is the first day of your period.
Changes in hormone levels trigger menstruation. If by the end of your previous menstrual cycle you aren’t pregnant, progesterone and estrogen levels drop quickly. Without these hormones, the lining of your uterus becomes unstable and starts to break down — and your period starts.
During your period, your uterus sheds the lining from the previous menstrual cycle. This includes tissue, mucus, and blood. Even though bleeding is a major feature of periods, most people only lose about a teaspoon of blood during a period.
Menstruation lasts between 3 and 7 days. During this time, your body is also “resetting” your hormone levels. These relatively quick hormone changes can cause symptoms like bloating, mood changes, and bowel habit changes.
Follicular phase
The follicular phase of the fertility cycle is a growth phase. It’s the longest phase of your menstrual cycle, lasting about 14 days if your cycle is 28 days long. If you have a longer menstrual cycle, it’s because you also have a longer follicular cycle. And if you have a shorter menstrual cycle, it’s because you have a shorter follicular cycle.
During this cycle, your estrogen levels start to rise. Estrogen sends signals to your uterus to rebuild its lining. It also tells blood vessels to grow toward the uterine lining so that blood and nutrients can reach the new lining. This healthy lining keeps growing so that it’s ready to receive an embryo.
High estrogen levels also make your cervical mucus more watery and loose. It’s easier for sperm to get through this thinner mucus and reach an egg. You might notice this as a slight change in your vaginal discharge. It’s typically the only symptom people experience during their follicular cycle.
Finally, estrogen levels also tell your ovaries that it’s time for one of your eggs to start to mature. Your ovaries start to mature this one egg while asking all the others to remain quiet. Once this egg is fully matured, it’s time to move to the next phase.
Ovulation
During ovulation, your body releases a mature egg that’s ready to be fertilized by sperm. Ovulation is the shortest phase — it lasts only 1 day. It’s the day you’re most likely to get pregnant during your fertility cycle.
A mature egg only survives for 12 to 24 hours. If it’s not fertilized by sperm, it will disintegrate. So it’s crucial to know when you’re ovulating if you’re trying to get pregnant.
Even though ovulation always happens 14 days before your next period, it can be hard to know exactly which day you’ll ovulate because menstrual cycles can be longer or shorter for some people. And your own menstrual cycle may vary in length from time to time.
People use a number of different methods to try and pinpoint this ovulation day including using at-home ovulation kits and tracking basal body temperature.
Even though you need to fertilize your egg within 24 hours, your ovulation day isn’t the only day you can get pregnant. Sperm can survive inside the body for 5 to 6 days. So your fertile window starts 5 days before you ovulate. You’ll want to time sex for this fertile window. If you’re using IUI to get pregnant, insemination happens when you have a positive ovulation test.
Some people may notice cramping when they ovulate. This usually only lasts a few hours. But, there’s no way to know for sure if you’re ovulating just by how you feel.
Luteal phase
The luteal phase is the last phase of the menstrual cycle. It always lasts 14 days. During this phase, your estrogen levels stay high. But your body also starts to make another hormone called progesterone.
Progesterone is the driving force of the luteal phase. It tells the uterus to stop making lining and start storing energy to help nourish the embryo that may be on its way. It also tells cervical mucus to thicken. This helps form a barrier between the vagina and cervix so that bacteria and viruses have a harder time getting into the uterus and near the embryo.
If the egg you released during ovulation gets fertilized, it becomes an embryo and then implants a day or two later into your uterine lining. This triggers your body to keep making progesterone and prepare to stay pregnant. If your egg isn’t fertilized, your body starts the fertility cycle all over again.
The bottom line
The menstrual cycle is divided into four phases: menstruation, the follicular phase, ovulation, and the luteal phase. Each cycle affects your ability to get and stay pregnant. Each person’s cycle is a little bit different, but in general you’re most likely to get pregnant in the 5 days leading to ovulation and on the day of ovulation. If you don’t get pregnant during a menstrual cycle, your body will restart the fertility cycle, beginning with the first day of your period.
Why trust our experts?
References
Bull, J. R., et al. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. Nature Partner Journals Digital Medicine.
MedlinePlus. (2022). Pregnancy - identifying fertile days.
National Health Service. (2023). Periods and fertility in the menstrual cycle.
Thiyagarajan, D. K., et al. (2022). Physiology, menstrual cycle. StatPearls.
Wilcox, A. J., et al. (2000). The timing of the "fertile window" in the menstrual cycle: Day specific estimates from a prospective study. The British Medical Journal.
