Basic Biology
Female Biology
A female is born with over 1 million eggs in her ovaries. This amount will never increase.
By the time she reaches puberty, she'll have about 300,000 eggs left. Of these, only about 300 eggs will mature and be released during her reproductive years. All eggs are stored in a woman's ovaries.1
Cycle of life
A woman's monthly menstrual cycle drives her ability to conceive. The cycle begins on the first day of a woman's period and typically lasts about 28 days. The first day occurs not with spotting, but when there is a regular flow.
The menstrual cycle has three distinct phases: the follicular phase, the ovulatory phase, and the luteal phase.
Natural Monthly Cycle
View the day-by-day animation of a woman's natural monthly cycle.
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Follicular Phase: Days 1–14
During the first 13 days of a woman's cycle, follicle stimulating hormone (FSH) causes her body to create a fluid-filled sac, or follicle in one of her ovaries. This follicle then produces a single mature egg. While this happens, the follicle releases another hormone called estrogen. Estrogen helps the cervical mucus at the entrance of her uterus to receive sperm. The endometrium also begins to thicken during this phase.
A rise in luteinizing hormone (LH) helps the egg reach maturity and on about the 14th day, triggers ovulation. At this time, an ovary releases the egg which then travels down a fallopian tube. When a couple has intercourse, a man's sperm tries to swim past the cervical mucus and into the fallopian tube where it can fertilize the egg. The best days for conception are generally days 11 to 17 — the ones just before and after ovulation.
Luteal Phase: Days 15–28
After ovulation, the hormones progesterone and estrogen help the endometrium receive the fertilized egg. The fertilized egg, or embryo then travels down the fallopian tubes to implant itself in the endometrium on about the 20th day of the cycle. There, it will develop into a fetus.
If the egg isn't fertilized, it passes through a woman's body on about the 26th day. The uterus then sheds its lining and a woman's period begins, starting a new menstrual cycle.
It is important to understand that 28 days is just an average. Not every woman has a 28-day cycle. The specific days of your Follicular and Luteal phases may be different. For more information, talk to your healthcare provider or call Fertility LifeLines™ at 1-866-LETS-TRY (1-866-538-7879).
Hormones' critical role
Sound complex? It is! Each step in the cycle is controlled by hormones. In order for a woman to conceive, the conditions must be just right. Hormones must be made in the correct amounts and at the right times. Any interruptions can disrupt a woman's ability to conceive. In addition, other factors may play a role in reducing fertility, such as age and lifestyle choices.
- 1. American Society for Reproductive Medicine. Age and fertility: A guide for patients. Birmingham, Alabama: American Society for Reproductive Medicine; 2003.
See for Yourself
Watch how a woman's monthly cycle progresses, day by day.
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