Reduced fertility in the industrialized world has been identified as a drop in male fertility. This page from a Belgian University identifies causes of reduced male fertility.
Note that it says that a "sensible" reason can be found in half the cases. That means that half the cases are unexplained--and of course, it begs the question to cite various forms of erectile dysfunction, as the cause of said dysfunction is not explained.
This article from CNN reports on a JAMA study which identifies pesticides as a cause of reduced fertility in women
. That begs the question of why there is reduced fertility in the industrialized world, given that pesticides are used in the so-called developing world, and often with fewer restrictions.
The "lowest-low" fertility rates, identified by demographers as a replacement rate of 1.3 to 1 or lower (one-point-three live births to every death) prevails in well over half of the nations of Europe. The replacement rate in the United States is at about two-to-one, which demographers attribute to immigration. The fertility rate in immigrant communities in the United States drops in the second and subsequent generation. The total fertility rate in East Asia is very low, with Singapore, Macau, Taiwan, Hong Kong and South Korea having some of the lowest rates in the world. This is also considered a serious problem in Japan, even more so because their replacement rate is not enhanced by immigration as it is in Singapore and Macau. These cities and nations are the most industrialized in East Asia.
This reduction in population growth in the industrialized world is not just attributable to social factors which delay willful reproduction. The relative infertility of citizens in the industrialized world, particularly men, was identified by demographers and epidemiologists decades ago. There is no consensus on the causes, and glib remarks by people who are not expert, who do not (apparently) understand how to properly evaluate evidence and who set themselves up as instant experts are, to put it with civility, silly and untrustworthy.
Anyone who claims to know why the birth rate in the industrialized world has been dropping for more than 50 years is peddling bullsh*t.