Every time we go to the OB/GYN, we hear the question, “When was the first day of your last period?” Although seemingly redundant, the duration of our menstrual cycle can reveal a lot about our health, and the changes our bodies go through each month. This allows us to get a better gauge at our fertility window, as the body prepares itself for pregnancy, but what does our cycle say about our ability to conceive naturally?

The Menstrual Cycle

The length of our menstrual cycle is determined by the number of days from the first day of bleeding to the start of our next period. This can tell us whether we have hormonal imbalances, and whether or not ovulation is occurring regularly. These imbalances can affect if and when ovulation occurs during our cycle, because without it, we can’t get pregnant.

Paying close attention to our menstrual cycle and the changes that take place in our body at this time can tell us a lot about our fertility. The cycle is divided into three phases, including the follicular phase, the luteal phase, and the menstrual period, and each phase may contain clues to our health.

“A normal menstrual cycle is typically 21 to 35 days,” Dr. Kecia Gaither, a double board-certified physician in OB/GYN and maternal fetal medicine at St. Luke's Medical Center in Kansas City, Mo., told Medical Daily. “When you start having irregularities, the periods are short or long, then that tells you the balance is out of whack.”

Consistently irregular cycles, whether short (less than 21 days) or long (more than 35 days), or skipping your period every few months, can have an effect on fertility, and the likelihood of a natural conception. Most cases of infertility are caused by problems with ovulation. Women with irregularities, also known as “abnormal ovulation,” might not be releasing a mature egg, or ovulating during their cycles, and this accounts for 30 to 40 percent of infertility cases.

Follicular Phase and Luteal Phase

The follicular phase ends at the start of ovulation, and this process can be short or long, playing the biggest role in determining the duration of our cycle. The follicle stimulating hormone (FSH), a female sex hormone, is released from the brain to stimulate the development of a single follicle which contains one egg. The follicle releases estrogen, which stimulates the growth of the uterine lining. The last five days of this phase, plus ovulation day, are our fertility window. This is when the likelihood of becoming pregnant increases without birth control.

The conclusion of the follicular phase is the beginning of the luteal phase, which starts on ovulation day — the day the egg is released from the egg follicle on the ovary. This can happen any time from day 7 to day 22 of a normal menstrual cycle.

Now, if the egg is fertilized by sperm and then implanted in the endometrium, a pregnancy begins. If the egg is not fertilized or does not implant, the endometrium begins to break down. This phase normally lasts 13 to 15 days, or two weeks before we’re expected to get our next period.

A normal menstrual period can last three to 7 days with most of our menstrual blood loss happening during the first 3 days.

Short Menstrual Cycle: Less Than 21 Days

When a menstrual cycle is less than 21 days, this means ovulation has not occurred. Typically, our menstrual cycle shortens as we get older, and our ovaries contain fewer eggs — tell-tale signs menopause is coming. Doctors can check hormone levels, so if FSH is really high, that’s a sign the eggs are decreasing, according to Gaither.

A decrease in the number of eggs available in the ovary leads the brain to release more FSH to stimulate the ovaries to develop a follicle. This causes the earlier development of the follicle and earlier ovulation, therefore, shortening the cycles. In some cases, bleeding can occur when ovulation doesn’t, which may appear as a shorter cycle.

Meanwhile, shorter, lighter periods could be an indicator of menopause for older women, or premature ovarian failure for younger women in their 20s. This condition refers to the loss of normal function of the ovaries before age 40. When the ovaries don't produce normal amounts of the hormone estrogen or release eggs regularly, infertility is a common result, according to the Mayo Clinic.

Long Or Irregular Menstrual Cycle

When a menstrual cycle is more than 35 days, ovulation is either not occurring, or it’s occurring irregularly. Usually, longer cycles are due to a lack of regular ovulation. In a regular cycle, the fall of progesterone is what causes us to bleed. However, if a follicle does not mature and ovulate, progesterone is never released, and the lining of the uterus continues to build due to estrogen. The lining becomes thick and unstable, eventually leading to bleeding. This can be unpredictable, and often very heavy, lasting a long period of time.

Being overweight or obese can also lead to heavy periods.

“You’re producing estrogen from two venues — ovaries and the extra fat. Body fat has the capability to produce a lot of estrogen, and all of this estrogen in the uterus could lead to uterine cancer,” said Gaither.

Estrogen encourages the growth of endometrial cells in the uterus, whereas progesterone inhibits it. When women have high levels of estrogen and low levels of progesterone over long periods of time, their risk for uterine cancer rises, according to Memorial Sloan Kettering Cancer Center.

Anovulation

Doctors use the term “anovulation” to describe when the ovaries do not release mature eggs regularly. This could be due to a number of factors, including irregularities with the thyroid gland or increasing levels of prolactin, which can inhibit the brain from communicating with the ovary. Polycystic Ovarian Syndrome (PCOS), a condition triggered by an imbalance of sex hormones, can cause failed ovulation and fertility, and increase the risk of miscarriage.

Silent Anovulation

However, women who have regular, normal-length periods do not always ovulate. A 2015 study found one-third of all seemingly normal menstrual cycles may be egg-less, known as “silent anovulation.” This occurs when a woman appears to have a regular cycle, but the ovaries release no egg. This can threaten a woman’s fertility, increasing her risk of bone less, early heart attack, and breast cancer. Stress is linked to silent anovulation, and researchers believe this is the body’s evolutionary response to stop us from getting pregnant when we’re amiss.

In addition to medical conditions, Gaither notes really strenuous physical activity, such as long-distance running, can lead to problems with fertility. A 2012 study found normal-weight women (BMI under 25) who engaged in vigorous exercise, such as running, swimming, and aerobics for five or more hours a week were 42 percent less likely to get pregnant than women who did not exercise at all.

Vigorous exercising can affect ovulation, and therefore, disrupt the menstrual cycle. The pituitary gland translates this activity as an inopportune time to stress the body with reproduction, inhibiting the signal to the ovary to promote ovulation. Extreme exercising could also affect implantation, or the egg’s ability to attach itself to the inside of the uterus.

Emotional, social, or physical factors can cause changes body-wide, affecting our cycle.

“Once you pay attention to your cycle, such as the heaviness, the length of the cycle; all of these things can impact your ability to conceive,” said Gaither.

Gaither advises women should seek help if they notice consistent irregularities with their cycle, or if they’ve been trying to get pregnant for at least a year.

Paying attention to what happens to our bodies every time of the month will help us understand our menstrual cycle, and what’s happening with our fertility.