I heard that fertility (overall, for like, everyone ever) is on the decrease. I looked this up and apparently, there are a few potential causes. Do you have any insight into this?
There are a couple of different ways we think about fertility.
The “fertility rate” is a common population statistic, defined as the number of children born per woman. From this perspective, the global fertility rate is certainly declining. The excellent ‘Our World in Data’ website has a great article here, which shows that the global fertility rate has halved since the 1950s.
In Australia, the fertility rate has halved since 1960, from about 355 children per 100 women, to 174 in 20171.
Even though fertility rate is the lowest it has ever been, there are now more births in Australia than ever before. This is because our population has grown so much over this time.
Things like migration, economic development, religion, education, and family planning all affect the fertility rate.
Another way to look at your question is to think of fertility as the opposite of infertility, which is commonly defined (for couples) as not becoming pregnant despite trying (having regular intercourse without contraception) for 12 months. The prevalence of infertility varies between countries but is generally accepted to be between 8-12% of all couples. There are no good data about how the prevalence of infertility might have changed over the years.
So, what are the factors that contribute to infertility?
1. A decline in fertility due to delayed parenthood
A woman’s fertility begins to decline from the age of 25-30, and the age at which women are having children is gradually getting older. In 2010, 18% of Australian women giving birth were under 25 years of age; in 2018, 13.2% were under 25. The proportion of women giving birth at age 30 or older grew from 54.9% to 60.2% over that time. A man’s age also affects the chances of his female partner becoming pregnant.
In addition to lowered fertility, the risk of pregnancy problems and poorer outcomes for babies is higher for fathers over 45, than for younger dads2, as is the likelihood of children having psychiatric disorders3. It seems that damage to the DNA in sperm, which increases as men age, has a strong influence not only on fertility but also the health of their children4.
2. Falling sperm counts
There has been a decline by as much as 50% in sperm counts over the last 80 years, and measures of sperm quality have also fallen5.
3. Disease-related infertility
In general, illness has a negative impact on an individual’s fertility. Health problems such as obesity and diabetes are associated with infertility, and the rates of these diseases are rising.
4. Environmental pollutants
Many endocrine-disrupting chemicals are common pollutants and may accumulate in the environment over time. These can affect male and female fertility5 through everything from the food we eat to the furniture we use.
5. Technology advancements
Once, before assisted reproductive technologies such as In Vitro Fertilisation (IVF), genetic causes of infertility could not be passed on. In fact, a decade ago the pioneer of intra-cytoplasmic sperm injection (ICSI) expressed concern that the technique was being overused and that genetic defects that would otherwise prevent conception, could be passed on.
Whether it’s population-based declines in fertility rates, or declines in individuals’ own fertility, the causes are many and varied and the solutions complex.
Answered by: Associate Professor Tim Moss
Associate Professor Tim Moss has PhD in physiology and more than 20-years’ experience as a biomedical research scientist. Tim stepped away from his successful academic career at the end of 2019, to apply his skills in turning complicated scientific and medical knowledge into information that all people can use to improve their health and well being.
Tim has written for crikey.com and Scientific American’s Observations blog, which is far more interesting than his authorship of over 150 academic publications. He has studied science communication at the Alan Alda Centre for Communicating Science in New York, and at the Department of Biological Engineering Communication Lab at MIT in Boston.