How To Calculate Your Fertile Window Using Your Menstrual Cycle

by Editors

How To Calculate Your Fertile Window Using Your Menstrual Cycle

When you’re trying to conceive, knowing your monthly fertile days can increase your chances for success. Learn how to find your fertile window based on your menstrual cycle and ovulation day.


In general, a woman’s fertile window is the day of ovulation (usually 12 to 16 days before the menstrual period begins) and the five days preceding it. For the average woman, that occurs somewhere between days 10 and 17 of her menstrual cycle. But the problem with using this general estimation is that few women are average. So the trick for every woman who is trying to conceive is to pinpoint her own individual most fertile time.


How To Identify Your Fertile Days Each Month

One way to determine your fertile days is to keep a record of your menstrual cycle (for eight or more months, if possible). Select your shortest cycle (say 27 days) and subtract 18 from it. The resulting number—nine in our example case—is your first potentially fertile day. Subtract 11 from your longest cycle (say 30 days) to get your last potentially fertile day—19 in our example case. So if the cycles you measured over several months were between 27 and 30 days long, you would be most fertile somewhere between days 9 and 19 of your cycle.

This is still a very wide window of opportunity. You can narrow it further by charting your basal body temperature (your morning temperature before getting out of bed). For most women, this ranges from 96 to 98 degrees Fahrenheit. When your temperature rises slightly (four- to eight-tenths of a degree), it usually means you have ovulated within the past 12 to 24 hours.

One more method of identifying your fertile period is to notice changes in your cervical mucus. This discharge can range from dry (following menstruation) to sticky (approaching ovulation) to wet, stretchy, and semi-transparent (during ovulation). Ovulation usually occurs from two days before to two days after the peak day of stretchy mucus.

While, as mentioned above, every woman’s cycle varies slightly, here’s a day-by-day account of what happens during an average 28-day cycle as the body prepares for a possible pregnancy.

Your Menstrual Cycle Day-by-Day

Days 1 to 5 of Your Cycle

If you are not pregnant, old dead tissue lining the uterus sloughs off during the beginning of your cycle and menstruation begins. Estrogen and progesterone levels are low. Body temperature is 96 to 98 degrees Fahrenheit.

Days 6 to 7 of Your Cycle

The hypothalamus, a brain structure that regulates the internal organs and controls the pituitary gland, secretes gonadotropin releasing hormone (GnRH). GnRH, in turn, tells the pituitary to release follicle stimulating hormone (FSH) and luteinizing hormone (LH), which cause the eggs, or follicles, in one of the ovaries to begin growing. As the eggs grow, they produce estrogen. Progesterone remains low. Cervical mucus is dry (through days eight or nine).

Day 8 (may extend to day 12) of Your Cycle

Secretion of estrogen increases, which causes the lining of the uterus to become thicker and generate a richer supply of blood vessels, preparing it to receive a fertilized egg. FSH and LH levels decline.

Day 10 of Your Cycle

Mucus becomes wet with cloudy, sticky, or whitish or yellowish secretions.

Day 12 of Your Cycle

Mucus becomes clear, slippery, and stretchy, signaling ovulation is near. You are most likely to become pregnant during this period. (Sperm survive for two to five days after intercourse, which is why sex now can lead to pregnancy even though ovulation is still several days away.)

Day 13 of Your Cycle

Estrogen rises dramatically, which boosts LH. LH stimulates the synthesis of progesterone, which causes FSH to rise. Within 12 hours of ovulation, body temperature rises between 4/10 and 8/10 of a degree and, if pregnancy does not occur, remains high until the next menstrual period.

Day 14 of Your Cycle

Estrogen falls sharply and LH surges, which causes the ovary to release the egg—also known as ovulation. The egg lives for about 12 to 24 hours.

Day 15 (may extend to day 24) of Your Cycle

The empty egg follicle—the corpus luteum—secretes increasing amounts of estrogen and progesterone to help prepare the uterus for a possible pregnancy. FSH and LH levels begin to drop.

Day 17 of Your Cycle

When your body temperature has stayed high for three days in a row, generally your fertile period is over.

Day 18 of Your Cycle

Cervical mucus becomes cloudy.

Days 21 to 22 of Your Cycle

Progesterone level peaks.

Day 25 of Your Cycle

The corpus luteum breaks apart. If the egg was not fertilized, progesterone begins to drop and cervical mucus is tacky. (If fertilization occurred, your progesterone level remains high.)

Day 27 of Your Cycle

Mucus is absent or dry.

Day 28 of Your Cycle

Estrogen level decreases and progesterone production rapidly drops. Mucus is thick. If you’re not pregnant, your period will begin tomorrow.

For more help with fertility prediction use our Fertility Calculator.