Fertility changes with age. Both males and females become fertile in their teens following puberty. For girls, the beginning of their reproductive years is marked by the onset of ovulation and menstruation.
It is commonly understood that after menopause women are no longer able to become pregnant. Generally, reproductive potential decreases as women get older, and fertility can be expected to end 5 to 10 years before menopause.
In today’s society, age-related infertility is becoming more common because, for a variety of reasons, many women wait until their 30s to begin their families. Even though women today are healthier and taking better care of themselves than ever before, improved health in later life does not offset the natural age-related decline in fertility.
It is important to understand that fertility declines as a woman ages due to the normal age-related decrease in the number of eggs that remain in her ovaries. This decline may take place much sooner than most women expect.
Ovulation and the Menstrual Cycle
During their reproductive years, women have regular monthly menstrual periods because they ovulate regularly each month. Eggs mature inside of fluid-filled spheres called “follicles.”
At the beginning of each menstrual cycle when a woman is having her period, a hormone produced in the pituitary gland, which is located in the brain, stimulates a group of follicles to grow more rapidly on both ovaries.
The pituitary hormone that stimulates the ovaries is called follicle-stimulating hormone (FSH). Normally, only one of those follicles will reach maturity and release an egg (ovulate); the remainder gradually will stop growing and degenerate.
Pregnancy results if the egg becomes fertilized and implants in the lining of the uterus (endometrium). If pregnancy does not occur, the endometrium is shed as the menstrual flow and the cycle begins again.
In their early teens, girls often have irregular ovulation resulting in irregular menstrual cycles, but by age 16 they should have established regular ovulation resulting in regular periods.
A woman’s cycles will remain regular, 26 to 35 days, until her late 30s to early 40s when she may notice that her cycles become shorter. As time passes, she will begin to skip ovulation resulting in missed periods.
Ultimately, periods become increasingly infrequent until they cease completely. When a woman has not had a menstrual period for 1 full year, she is said to be in menopause.
As women age, fertility declines due to normal, age-related changes that occur in the ovaries. Unlike men, who continue to produce sperm throughout their lives, a woman is born with all the egg-containing follicles in her ovaries that she will ever have.
At birth there are about one million follicles. By puberty that number will have dropped to about 300,000. Of the follicles remaining at puberty, only about 300 will be ovulated during the reproductive years.
The majority of follicles are not used up by ovulation, but through an ongoing gradual process of loss called atresia. Atresia is a degenerative process that occurs regardless of whether you are pregnant, have normal menstrual cycles, use birth control, or are undergoing infertility treatment. Smokers appear to experience menopause about 1 year earlier than non-smokers.
A woman’s best reproductive years are in her 20s. Fertility gradually declines in the 30s, particularly after age 35. Each month that she tries, a healthy, fertile 30-year-old woman has a 20% chance of getting pregnant.
That means that for every 100 fertile 30-year-old women trying to get pregnant in 1 cycle, 20 will be successful and the other 80 will have to try again. By age 40, a woman’s chance is less than 5% per cycle, so fewer than 5 out of every 100 women are expected to be successful each month.
Women do not remain fertile until menopause. The average age for menopause is 51, but most women become unable to have a successful pregnancy sometime in their mid-40s. These percentages are true for natural conception as well as conception using fertility treatment, including in vitro fertilization (IVF).
Although stories in the news media may lead women and their partners to believe that they will be to able use fertility treatments such as IVF to get pregnant, a woman’s age affects the success rates of infertility treatments.
The age-related loss of female fertility happens because both the quality and the quantity of eggs gradually decline.