Many women don't fully understand that they're fertile for only a few days around ovulation. Once ovulation occurs, the hormone progesterone is secreted, which suppresses the release of other eggs until the following cycle. Once that fertile window has closed, it remains shut until the next time she ovulates.

One of the most prevalent myths about fertility is that ovulation always occurs on day 14. If this were true, there would be virtually no need for birth control, since couples could simply avoid intercourse around that one day. The truth is, the timing of ovulation can vary from one woman to another, and even from one cycle to the next in the same woman.

Fertilization doesn’t happen at the same time as sex. Sperm can survive inside a woman’s body for up to five days after intercourse, and fertilization can happen anywhere in that window — assuming an egg arrives in time.

As chronological age increases, fertility declines. A female is born with all the eggs she will ever have (one to two million), and the number declines to 300,000 to 500,000 at puberty. Age delivers the harshest blow to fertility, causing a mild decline at age 35, a precipitous drop at age 38, and another whopping one at age 40.

Being significantly overweight or underweight can decrease a woman's chance of getting pregnant by disrupting her proper balance of hormone levels. So can excessive exercise or athletic training and heavy alcohol use.

There are many factors in both men and women that can affect fertility. Cigarette smoke is toxic to eggs and embryos, and increases the risk of miscarriage. Sexually transmitted infections can make it more difficult for both men and women to conceive, and some can cause serious pregnancy complications. Health conditions (like endometriosis), medication, recreational drug use, and heavy caffeine consumption can also impact fertility.

Even if you choose not to have children, it's important to pay attention to your fertility status. Associations have been found between reproductive variables in women, such as premature egg loss, primary ovarian insufficiency, or early menopause, and risk of developing cardiovascular disease.

The association between a person's reproductive health and future overall health is so strong that fertility status could be considered a sixth vital sense, although more research needs to be done before a definitive answer can be made.