Your menstrual cycle is the series of hormone-driven events that prepares your body to get pregnant and carry a baby. This cycle follows a process that’s divided into four distinct phases:

Menstruation

This is the first, but also in some ways the last, phase of your menstrual cycle. It’s when the thickened lining of your uterus sheds during your monthly period. Menstruation can last from three to seven days, depending on the length of your cycle.

Follicular phase

This starts on the first day of your menstrual period and ends when you start to ovulate. During this phase, the egg-containing pods called follicles ripen and one of the eggs matures.

Ovulation

This phase happens when the ovary releases that mature egg down the fallopian tube on its way to fertilization. This is the shortest phase of the cycle, lasting just 24 hours.

Luteal phase

In this phase, the follicle that released the egg produces hormones that thicken and ripen the uterus to ready it for pregnancy.

Every woman’s menstrual cycle is unique. The length of each cycle and its phases can vary based on your age and other factors.

If you’re trying to get pregnant, it can help to know whether your follicular and luteal phases are long or short, and when in your menstrual cycle they happen. Problems with these phases could affect your fertility. Let’s take a closer look at the follicular phase.

The follicular phase starts on the first day of your period. Typically, it takes up the entire first half of your menstrual cycle.

This phase begins when your body’s hormone control center, the hypothalamus, sends a message to the pituitary gland at the base of your brain. The pituitary then releases follicle-stimulating hormone (FSH).

FSH stimulates your ovaries to produce 5 to 20 tiny pods called follicles. Inside each follicle sits an immature egg. These follicles grow during this phase of your cycle.

Eventually, one of these follicles becomes dominant. The other follicles start to wither away and are reabsorbed into your body.

The follicle with the ripening egg increases your body’s production of estrogen. Higher estrogen levels make your uterine lining grow and thicken. The lining becomes rich in nutrients to prepare for a possible pregnancy.

Rising estrogen levels also send a signal to your pituitary gland to slow FSH production.

Meanwhile, levels of another pituitary hormone called luteinizing hormone (LH) surge. The rise in LH halts estrogen production and starts the process of ovulation, the next phase in the cycle.

The follicular phase is often the longest part of your menstrual cycle. It’s also the most variable phase. It begins on the first day of your period and ends when you ovulate.

The average length of the follicular phase is 16 days. But it can last anywhere from 11 to 27 days depending on your cycle.

The length of your follicular phase depends in part on the amount of time it takes one dominant follicle to emerge. When the follicle is slow to mature, this phase will last longer. Your whole menstrual cycle will also be longer as a result.

A long follicular phase means that it takes more time for your body to ovulate. Using birth control pills for a long time can lengthen your follicular phase. Low vitamin D levels have also been linked to a longer follicular phase.

Women with a long follicular phase are just as likely to get pregnant as those with a statistically more normal follicular phase. Having a longer cycle shouldn’t affect your fertility.

Having a short follicular phase could impact your likelihood of conceiving, though. It may be a sign that your ovaries are aging and you’re getting closer to menopause.

The follicular phase may start to get shorter when you’re in your late 30s, even if you still get a monthly period. Hormone levels change during this time. Your FSH levels still rise, but your LH levels stay low. This causes a follicle to ripen too quickly. The egg inside that follicle may not be mature enough or ready to fertilize. This makes pregnancy more unlikely.

Tracking your basal body temperature can help you figure out on which days of the month you’ll have the best odds of conceiving. Your basal body temperature is your lowest temperature when you’re at rest.

To measure basal body temperature, keep a thermometer at your bedside and take your temperature upon waking, before you even get out of bed. This should be done at the same time each morning.

In the follicular phase of your cycle, your basal body temperature should be between 97.0 and 97.5°F (36°C). When you ovulate, your temperature will rise and remain higher during the luteal phase, confirming that the follicular phase is over.

The follicular phase is the stage of your menstrual cycle when your body is preparing to release an egg. This is a necessary process for pregnancy. Once the egg is released, the follicular phase is considered over. For many women, this generally happens halfway between the first day of one menstrual period and the first day of the next menstrual period.

Menstrual cycles follow a general pattern, but the length and duration of a woman’s cycle can vary. If you’re tracking your cycle and you don’t ovulate when you think you should, don’t panic. Talk to your doctor. They can diagnose any potential issues with the follicular — or any — phase of your cycle.