Doctors Explain Why Women’s Fertility Window is So Short

The average number of childbearing years a human female has where she could biologically conceive a child is about 30 years – or approximately35% of a woman’s lifespan.  This includes the time between a young woman’s first period and when she reaches menopause (the end of a woman’s menstrual cycles and reproductive ability).

However, in reality, the fertility window is much, much shorter – about 1/3 of that. Economically, medically, and socially, there are significant advantages for women to wait until the age of 20 (or later) to become pregnant. And pregnancy after the age of 35 not only becomes more difficult to achieve but is more dangerous for both mother an infant.

So, in humans, the optimal fertility window is actually only 15 years long – only 19% of a woman’s lifespan. And this is among the shortest fertility window of all mammals on the planet!

In this article, Austin TX fertility expert Dr. Ruthie Harper, MD explores the multitude of factors influencing this very short fertility window in human women.

When Does the Female Fertility Window Open?

Based on the National Health Statistics Reports derived from data in the National Survey of Family Growth (NSFG) the average median age for the start of female’s period decreased from 1995 (12.1 years) to 2013 through 2017 (11.9 years old). While girls are ovulating and therefore technically fertile at this young age, they are clear not physically, emotionally, or even financially candidates for motherhood for many more years.

In fact, The American College of Obstetricians and Gynecologists (ACOG) recommends that women wait until they are at least 20 years old before having a baby. This is because the teenage body is still developing, and pregnancy can increase the risk of complications for both the mother and the baby.

However, in the U.S., according to census data, the number of births to women under the age of twenty-five has dropped significantly since 1990 – and even an increase in births to women over thirty-five has not offset this decline in childbearing. In 2020, 96% of women who gave birth were over the age of 19, and 60% were aged 25 or older.

When Does the Female Fertility Window Close?

The average age for menopause in the United States is 51, but it can occur earlier or later in some women. However, it is important to note that fertility declines significantly with age, even before menopause. This is because the quality and quantity of eggs produced by the ovaries decreases with age. As a result, it becomes more difficult for women to conceive naturally as they get older.

The risk of pregnancy and delivery complications also increases with age. For mothers over the age of 35, the risk of miscarriage, stillbirth, and birth defects is higher. Additionally, older mothers are more likely to have health problems such as gestational diabetes and preeclampsia.

Less than 14% of births in the US are to women age 35 or older. And less than 3% of babies are born to women age 40 or older.

Of course, every woman is different, and some women may be able to have healthy babies safely at a younger or older age. However, it is important to be aware of the risks involved and to talk to fertility expert Dr. Ruthie Harper in Austin, TX – or your doctor – about the best time for you to start a family.

It is also important to note that there are a number of additional factors that can affect a woman’s fertility, in addition to her age, including overall health, and lifestyle choices. Smoking, excessive alcohol consumption, and obesity can all reduce fertility. Additionally, certain medical conditions, such as endometriosis and PCOS, can also make it difficult to conceive.

Age and Percent of Delivery in the US in 2020

This information on the number of births by age group is based on the 2020 US census data. Data has been rounded and is approximate, but gives a good picture of the current age of childbearing in the US.

Age group (US)	Number of births (US) rounded	Percentage of total births approximate
15-19	51,000	4%
20-24	391,000	34%
25-29	335,000	29%
30-34	215,000	18%
35-39	127,000	11%
40-44	22,000	2%
45 & over	6,000	Less than 1%

Why the Fertility Window is So Short in Humans

Unlike most other female mammals, who can maintain the ability to reproduce throughout their lives, human females experience a marked decrease in fertility as they reach their mid-30s and beyond. Chimpanzees can reproduce for up to 80% of their lifespan. And elephants reproduce for nearly 60% of their life. But humans are “able” to reproduce for 20% to 30% of their time on earth.

The phenomenon of female fertility decline at midlife has long intrigued biologists and researchers. Furthermore, ovaries in women age at a rate much faster than other organs in the body. This puzzling aspect of human biology has led to various theories, though none are conclusive.

There are a number of theories about why human females experience such a dramatic decline in fertility at midlife. One theory is that it is an evolutionary adaptation. According to this theory, it was beneficial for early humans to have a limited reproductive lifespan. This is because childbirth is risky, and early humans often faced harsh environmental conditions. By having a limited reproductive lifespan, women could avoid the risks of childbirth later in life, when they were more likely to die from other causes.

Other scientists postulate that rapid ovarian aging in human women may be linked to oxidative stress and DNA damage. The ovaries, responsible for producing eggs, are particularly vulnerable to the damaging effects of oxidative stress over time. This theory suggests that the high metabolic activity in the ovaries might expose them to more oxidative damage compared to other body organs.

While these theories offer valuable insights into the biology of female fertility decline and ovarian aging, the exact mechanisms behind these phenomena remain elusive. But whatever the evolutionary or biological reason, there is no denying that the window for having a child is remarkably short. 

Fertility Doctor | Austin TX

Bottom line: the average number of childbearing years a human female safely and fully has is only about 15 years. This is accounting for too young (teens) and risk of complications (too old). (Please note that this is just a general guideline. It is important to talk to your doctor about your individual needs and risks before making any decisions about pregnancy.)

In light of this sobering statistic, women who are considering expanding their family need to be empowered to take full control of their medical, physical, mental, environmental, dietary, and lifestyle factors that will set the best stage for fertility and improve the chances of conception.

If you are concerned about your fertility, it is important to talk to a fertility expert like Doctor Ruthie Harper, MD in Austin, TX. She can assess your individual risk factors and recommend steps you can take to improve your chances of getting pregnant. Dr. Harper can also help you restore your overall health – so that your body is “in balance” and better equipped to conceive a child safely and naturally.

Fertility Doctor | Austin TX: 512.343.9355

Resources:

Witt, Emily (2023). The Future of Fertility: A new crop of biotech startups want to revolutionize human reproduction. New Yorker Magazine April 17, 2023

Martinez, Gladys Ph.D. (2020) U.S. Department of Health and Human Services Centers for Disease Control and Prevention, National Center for Health. Statistics Trends and Patterns in Menarche in the United States: 1995 through 2013–2017. National Health Statistics Reports Number 146, September 10, 2020.

Emera, D. (2019). Human fertility and the menopause transition: A comparative evolutionary perspective. Nature Reviews Endocrinology, 15(9), 571.

Stearns, S. C. (2000). Life history evolution: Theories and analyses. Princeton University Press.

Walker, R. L. (2007). An introduction to human evolutionary genetics. Wadsworth Publishing.