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Understanding a Woman’s Biological Clock and Its Relationship to Fertility

This article examines the science behind a woman's biological clock and how women going to college and entering the workforce has changed our perception and timeline of having children.

July 15, 2020
Maura McLay

At some point in a woman’s life she hears the comment, “Your biological clock is ticking!” This can come from family, friends, and even partners when a woman has reached child bearing age and the pressure to have children becomes real. 

In the past, the ideal life of a woman would be to have children and oversee matters of the household. But today, women have taken a shift towards focusing on education and having a career. Since the end of WW2, women’s participation in the workforce has grown -- from 32.7% in 1948 to 56.8% in 2016. Also in 2016, 40% of women in the workforce had earned a college degree, whereas in 1970, only 11% of women had a college degree. 

In 2018, the average age of first time mothers climbed to 26.9, compared to 21.4 in 1970. This shift in age correlates to the shift in women’s focus -- women are now focused on their education and careers. But still, this does not halt the pressure of the biological clock and the window women are most likely to get pregnant. 

In this article we will look at the science behind a woman’s biological clock and explain how it works. We will discuss: 

  • How Fertility Changes Over Time 
  • What This Means for Starting a Family

What is My Biological Clock? 

The Science Behind Female Fertility

Graph that shows fertility in relation to age
Female fertility curve of fertility based on age. Image courtesy of The Conversation

A team of researchers from the University of Copenhagen have identified some of the factors that affect women’s fertility -- largely due to chromosome errors that arise depending on a woman’s age. Their study looked at 3,000 egg cells of women between the ages of 9 and 43. 

When a woman is born, she is born with all of the eggs she will have during her entire lifetime. Only these eggs are not mature until she begins her menstrual cycle. So, a woman’s biological clock starts off very poor during teenage years, begins to rise during the 20’s and starts to decline again when a woman reaches her 30’s. 

The researchers found that a kind of molecular glue makes the chromosomes stick together. Then when the immature eggs cells start to reach maturity, the chromosomes start to divide. The issue with older women is the molecular glue may prematurely break down. When this happens, the hereditary material wears away -- leading to chromosome errors such as Down, Turner, and Kleinfelter syndrome.

In teenage girls they found a greater occurrence of chromosome errors during the maturing of egg cells. They found that some egg cells did not reach a sufficient level of maturity and --- therefore, the body would be more likely to expel these eggs cells without being fertilized. 

They then found that as women began to enter their 20’s, the eggs began to increase in health and decrease in potential for chromosome errors. After the 20’s, different types of chromosome errors began to appear. 

Despite these factors, scientists do not have a definitive reason as to why female fertility rises, peaks, and falls at different points during a woman’s lifetime. One reason may be a bodily function that protects young girls from becoming pregnant until their bodies are fully developed and able to successfully carry a pregnancy without significant risks or complications.

Another factor may be something called the “grandmother hypothesis.” This theory suggests that female fertility peaks in older women because they are supposed to take on a new role as a grandmother. This is to support their own children when it comes time for them to produce offspring of their own. 

The researchers state that if we are able to better understand our biological clocks, we may be able to control the breakdown of egg cells. Eggs, rather than the uterus, have been identified as the cause of fertility issues. 

What Does This Mean For Having Children? 

Do I Have to Choose Between a Career and Starting a Family? 

Mother working on laptop with child sitting in lap
It is possible to both have a successful career and be a mother. 

The reality is that the biological clock does exist, but the way much of society looks at the biological clock is outdated. The truth is -- by 45 mostly all women will be entering menopause and are no longer be fertile. Women’s fertility begins to significantly decline around 37, with a large drop after the age of 40. 

The biological clock doesn’t simply stop at age 35 -- this is a common misconception held by society. It is true that fertility levels begin to decline, but having children can still be a possibility -- especially with fertility treatments and options. 

It is possible to go to college, start a career, and still have a family. There is a significant amount of time to achieve all of those dreams, so a woman shouldn’t feel pressured to have to choose between one or the other. 

If you have waited until you are older to try for a pregnancy there are some options you may choose. One option is having egg cells frozen when you are younger -- as this allows for a woman to preserve the quality and quantity of her eggs. This has often been described as a way to “pause” the biological clock. But proceed with caution because there is no guarantee of successful in-vitro implantation -- this depends on your body. 

Another option is looking for an egg donor. Egg donors can lead to successful pregnancies through IVF treatments. A woman in her 50’s has the same likelihood of a successful outcome from egg donor IVF as women in their 30’s and 40’s. 

The bottom line is focus on what you want. Some women may feel pressured to have children even if they don’t want them -- the choice is yours. If you want to have a career and have children, you can do this too -- 70% of mothers with children under the age of 18 are active in the workforce. Additionally, 40% of mothers with children under the age of 18 are the primary earners for their households. If you are older and want to have children, don’t forget to seek out fertility specialists to better understand your options. 


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