THE DIFFERENCE BETWEEN BIRTH RATES AND FERTILITY RATES

Briefing: A recent article in The Wall Street Journal attributing “fertility rates” to everyday chemicals caught our attention because it was emblematic of the confusion surrounding the two terms: birth rates and fertility rates. Because a correct understanding of how these two terms differ is of crucial importance in writing and reading about these rates, we decided to use this article as a teaching opportunity.

Nobody can deny that birth rates are declining in almost all economically advanced countries and, to lesser degrees, even in many developing countries as well. Declining birth rates, however, do not automatically mean declining fertility rates, even if the public (including media) unfortunately uses those two terms often interchangeably (just Google the question “Are fertility rates declining?”).

A recent article in The Wall Street Journal attributed in its title a decline in “fertility rates” to everyday chemicals (1). The anecdotal case report offered as an example, however, reveals the whimsiness of the claim: A 30-year-old nurse anesthetist failed for a few months (!!!) to conceive, and her doctors told her that her ovaries “weren’t functioning properly” (???). A Facebook group – and scientific papers (???) she subsequently looked up convinced her that synthetic chemicals found in everyday consumer products were likely to blame.

As a consequence, she decided to reduce her exposure to such chemicals and – lo and behold – she conceived two years later (!!!) with help of an intrauterine insemination (!!!). What a surprise! And we hope that our readers will understand the symbolism of the exclamation and question marks in parentheses, pointing out the absurdity of the alleged associations.

So, what is really going on? Birth rates, indeed, are almost universally declining. In the U.S. this has been the case since 2007, reaching historical lows again in 2024. According to the CDC, they declined in only one year from 2022 by 3%. The average U.S. female, therefore, currently has only 1.6 children, when a country needs a birth rate of 2.1 children per woman to maintain its population. And having birth rates below replacement levels, of course, creates considerable economic problems because it means a shrinking workforce and a relative increase in the aging population, resulting in fewer working people having to take care of more already retired people (now you know why Elon Musk is planning to produce humanoid robots!). The world’s total birth rate has fallen from 4.8 children in 1950 to 2.2 today, with the only continent still producing a birth rate above replacement being Africa.

Like almost always, Musk, therefore, has been absolutely correct: the world does not suffer from over- but from under-population!

Why the drop? Likely the principal cause is the close inverse association between development and prosperity and birth rates. But other causes may, of course, also contribute. Likely the most important among those is significant delay in childbearing by women who, now, are usually in the work force. Such delays have two potential consequences. First, to spontaneously conceive takes increasingly longer and more often fails with advancing female age. And second, the remaining reproductive lifespan of women is shortened, allowing for fewer pregnancy attempts.

Though declining fertility rates of women and men may obviously also contribute their effects on overall birth rates it is—at worst—only small. Which brings us to the main point of this commentary: While fertility rates in the world may also be declining, this development is relatively insignificant in comparison to the decline in birth rates. To clearly differentiate between birth and fertility rates, therefore, is essential. In contrast to birth rates, a decline in fertility means that either a woman or a man does not spontaneously conceive within a defined time period and whether—considering this definition—we are becoming less fertile is somewhat unclear. There exists, indeed, at least some evidence that certain environmental agents may be culprits in adversely affecting female and/or male fertility (more on that below).

As The New York Times article points out correctly, many of the suspected substances are so-called “endocrine disruptors,” so-called because they mimic and/or block hormones responsible for reproductive functions (one reason why we here at our IVF center do not like Chinese herbs in women receiving fertility treatments, which often contain especially estrogen measurements-affecting substances). But they can also be found all over our environment, with especially plastic packaging having attracted lots of attention recently, but also cosmetics, toys, sofa coverings, etc. PFAS (manmade polyfluoroalkyl substances) have been tagged by the U.S. Environmental Protection Agency as “possible” (!!!) interference with reproduction, and they are everywhere, from drinking water to non-stick cookware, and takeout containers in every home.

The article further noted that higher levels of PFAS in blood were associated by researchers at New York’s Mount Sinai Institute for Climate Change, Environmental Health, and Economics with significant reductions in likelihood of pregnancy and live births. Trying to track down this paper, we were unsuccessful beyond finding a note from the National Institute for Environmental Health Sciences of June 29, 2023, which funded the Mount Sinai center, reporting these alleged findings of the Mount Sinai investigators in a study of women of reproductive age in Singapore (2). 

Without offering any detail, the article also claims that studies have demonstrated that PFAS can delay puberty, and increase the risk for PCOS and endometriosis, while so-called phthalates allegedly reduce oocyte yields in IVF cycles.

Returning to the nurse anesthetist featured in Wall Street Journal article, we would not only argue that her case offers absolutely no evidence for infertility caused by any of above discussed environmental substances but – if there, indeed, was an environmental effect at all – it likely came from the gases nurse anesthetists are, of course, exposed to at all times in the operating room, and even that is questionable.  

References:

1.   Chaudhuri S. The Wall Street Journal. December 14, 2024. https://www.wsj.com/health/fertility-chemicals-science-bc0964a1

2.  National Institute of Environmental Health Sciences. June 29, 2023. https://www.niehs.nih.gov/research/supported/centers/core/spotlight/fertility#:~:text=The%20team%20reported%20that%20higher,who%20were%20trying%20to%20conceive.