Your Menstrual Cycle, Your Fertility

A typical menstrual cycle is 28 days long, with menstruation occurring on day 1.

One way to determine your individual fertile period 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—is your first potentially fertile day. Subtract 11 from your longest cycle (say 30 days) and you get 19. This marks your last potentially fertile day. 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.

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, it 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. The mucus ranges 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.

Days 1 to 5

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

Days 6 to 7

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)

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

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