Many want to know if the dystopian future Atwood created in which the world has largely become infertile, is in fact possible. And are we on our way there already?
What this latest study found
The recent paper that hit headlines all over the world highlighted the issue of declining sperm numbers in Western men.
The study is a meta-analysis, which gathers together similar studies and combines the results. Each of the studies in the analysis has different men assessed at different times by different researchers. This means, as a whole, it is not as powerful as a study examining the same men over time. And many of the individual studies assessed have their own problems.
So is fertility actually declining?
The current estimate is that Western men produce 50 million sperm per millilitre in an ejaculate, which is lower than previously. However, only one sperm is needed to fertilise an egg, so 50 million sperm per ml suggests human males don’t have a problem just yet.
There are data indicating that from below 40 million sperm per ml there is a linear relationship between sperm numbers and probability of pregnancy. The World Health Organisation (WHO) suggests 15 million per ml sperm is a minimum to be considered fertile. The minimum is based on men who have successfully fathered a child in the last 12 months. By definition, 5% of the men with numbers below 15 million per ml will still be able to reproduce.
For females the issue that needs to be understood is that there is already a small window of time women are fertile, and this is decreasing as women are more educated and career-focused.
Women have their highest number of eggs when they are still a fetus in their mother’s womb. About one sixth of the eggs are left at birth and by puberty the number is 500,000 eggs or less. From puberty until 37 years of age there is a steady decline from 500,000 to 25,000 eggs. After 37 years, the rate of decline increases and by menopause (average age of 51 in the US) only 1,000 eggs remain. It’s important to realise these are average numbers and there is no guarantee a woman will have 25,000 eggs at 37.
The other issue is quality. Chromosomal issues (such as Down’s syndrome – where a person has three copies of chromosome 21 instead of two) increase with maternal age. IVF is seen as a way of rescuing fertility, but the success rate of 41.5% is for women younger than 35, and measures pregnancies, not live births. By 40 years old, that success rate is 22% and by 43 years it’s 5%.
In short, the situation for women is not great, but the numbers are not changing with time (estimates of fertility from 1600 to 1950 don’t differ).
Male sperm numbers show males do not have a fertility problem, but declining numbers could signify health problems
What is affecting fertility today?
The key determinant in women’s fertility is education – not individuals’ education but that of the community as a whole. If your community becomes educated, your fertility declines, as women become educated and less likely to have children in their youth.
Choosing to delay having a child is not the only issue. Lifestyle choices matter. We know smoking, alcohol and obesity all affect the number and quality of eggs a woman has. As a female has all the eggs she will ever have when she is in her mother’s womb, the mother smoking will affect those eggs. Smoking in pregnancy is declining slowly (from 15% in 2009 to 11% in 2014) but is still very high in the Indigenous population (45%).
Smoking and alcohol are said to be major factors contributing to male sperm numbers but the evidence is limited by the nature of the studies. The effects of obesity and stress have the clearest evidence. For example, increased levels of anxiety and stress have been associated with lower sperm count. Life stress (defined as two or more stressful events in the last 12 months) has been found to have an effect, but not job stress.
For men, the numbers themselves represent a blunt measure of fertility. It’s the quality of the sperm produced that’s of concern. The WHO minimum is that only 4% of male sperm need to be of good appearance to be considered fertile. It’s not really possible for us to be able to tell which of many factors may be influencing sperm appearance.
Problems with studying fertility
While we can talk about what research says on fertility, there are a few inherent problems with researching in this field. Most of the data we have on sperm count come from two sources: men attending an infertility clinic, and those undergoing a medical prior to military service. The first is restricted to those who likely already have a problem. The second is limited to one age group.
Meta-analyses, which combine the results from lots of studies, are limited to those all using the same tools and approaches so they can be compared. As a result, a large meta-analysis that suggested smoking is detrimental was limited to men attending an infertility clinic, which would indicate many of them are likely to be infertile anyway.
Another big study used conscripts in the US and Europe but failed to find an association between fertility and alcohol consumption. This is because it only assessed the alcohol consumed the week prior to the medical – and most recruits probably wouldn’t be out drinking in the days leading up to their medical.
So could we become extinct?
The reproduction rate is below that required for total population replacement in the US, Australia, and many other countries. But the human population in total is still growing as it ages.
The start of this millennium also represented the time when births for women aged 30-34 overtook those in the 25-29 age group, and the 35-39 age group overtook the 20-24 age group. Teenage pregnancy (15-19 years) is now level with older mums (40-44) in Australia.
The quality of the sperm and egg is more important than the numbers. While we are still investigating what quality means to future generations, we do know that infertility represents a predictor of increased death rates. Men diagnosed with infertility had a higher risk of developing diabetes, ischaemic heart disease, alcohol abuse and drug abuse.
Ultimately it’s not a numbers game but a quality game. This is true not just for the chances of having a child but having a healthy child. More immediately, fertility is a predictor of general health. While it does not appear that we are going to be extinct soon (at least not through reproductive failure), sperm quality could be a signal of wider health problems and should be investigated further.
Shaun Roman, Senior Lecturer, University of Newcastle
This article was originally published on The Conversation. Read the original article.
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