More and more parents delay having their first child, and in industrialized countries, childlessness rates are going up. This is due to socioeconomic, cultural, and environmental factors, as well as personal choice. New research, however, shows genetics might also play a role.

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Genetics might influence our reproductive choices.

Women are increasingly choosing to have a child later in life. In the 1970s, the average age at which women decided to have their first child was 24, whereas in 2012, it was 29.

An increasing number of women also choose not to have any children at all. In the mid-1970s, 10 percent of American women aged 40-44 had never had a biological child. By 2005, this had surged to 20 percent, decreasing to 15 percent in 2015.

In industrialized countries, both parents are choosing to have children later in life, which affects the number of children they can have and their reproductive ability.

Both women’s and men’s age are important factors in the ability to conceive. By the age of 40, a woman’s chances of conceiving have decreased by 50 percent.

Men’s fertility starts to decrease significantly after the age of 40, and the risk of miscarriage is twice as high for women whose male partner is aged over 45 than for those with partners in their 20s.

As an increasing number of people are putting off having children, researchers have looked at the cultural and socioeconomic factors that may be responsible for these choices.

However, until now, there was no significant research on the genetic or biological factors that might influence this trend. A new study aims to fill this gap by looking at how DNA might influence our reproductive behavior.

The study looked at the entire human genome to see if there was a connection between our genetic variants that make us particular individuals, our age at first birth, and the number of children born.

For the first time, researchers have managed to identify parts of DNA that influence not only the age at which parents decide to have children, but also the total number of children we decide to have during our lives.

The research consisted of a meta-analysis of 62 existing studies. In total, researchers collected information from over 600,000 men and women. They looked at 238,064 men and women for their age when they had their first child, and 330,000 men and women for the number of children they had.

They also gathered data from 250 sociologists, biologists, and geneticists from institutions worldwide.

The study was led by Nicola Barban, from the Department of Sociology and Nuffield College at the University of Oxford in the United Kingdom, and the results have been published in the journal Nature Genetics.

Researchers found 12 areas of the DNA code that are related to the age when we have our first child and the total number of children we have.

Variants in these areas combined can help predict around 1 percent of the time when men and women have their first child. They can also help predict 0.2 percent of the variability in the number of children we decide to have.

Our genes do not determine our behavior, but for the first time, we have identified parts of the DNA code that influence it. This is another small piece to understanding this very large jigsaw puzzle.”

First author Nicola Barban

By studying these 12 areas and their functions, researchers have been able to identify 24 genes that play a role in the time we decide to have children. Some of these genes have already been connected to infertility.

The data might seem very small, but as the authors point out, being able to predict 1 percent of human reproductive behavior is significant enough to be helpful for reproductive health experts.

The study points out that these DNA variants are also linked with other characteristics of sexual development, such as the age at which girls start menstruation, when the voice breaks for young boys, or when women start menopause.

The analysis revealed that women who tend to have a later menstruation and a later menopause onset are also likelier to postpone having a child, based on their DNA variants.

This research shows that in addition to social and cultural factors, there is also a biological basis for our reproductive choices.

For the first time, we now know where to find the DNA areas linked to reproductive behavior […] One day it may be possible to use this information so doctors can answer the important question: ‘How late can you wait?’ based on the DNA variants. It is important to put this into perspective, however, as having a child still strongly depends on many social and environmental factors that will always play a bigger role in whether or when we have babies.”

Lead author Prof. Melinda Mills

The new discovery might one day help predict infertility. This could provide valuable help to families or single parents who are trying to conceive.

DNA research of this kind might also increase the effectiveness of assisted reproductive technology (ART).

ART is currently invasive and sometimes risky. For example, it may cause multiple births, which can pose risks to both the mother and baby.

Although the use of ART is still relatively rare compared with those who choose to conceive naturally, the use of technologies to aid infertility has increased steadily since the birth of the first baby conceived with ART in 1981. In fact, the Centers for Disease Control and Prevention (CDC) report that its use has doubled over the past decade.

According to the CDC, in 2012, ART accounted for 1.5 percent of all infants born in the United States.

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