The researchers also broke down fertility rates and average maternal age by non-Hispanic white, non-Hispanic black, and Hispanic women. (Native American, Asian, and mixed race weren't accounted for.) Among all three groups, there was a downward trend in fertility rate and an upward trend in average maternal age. The largest decline was seen among Hispanic women, though they had the highest rates overall (26 percent in rural areas, 29 percent in small or medium metro areas, and 30 percent in large metro areas).
So, why the change? It is not exactly clear but there are several theories, from lower sperm counts to terrible maternal leave policy. As for the rise in average maternal age, it could be that women are simply choosing to have kids later so they can spend more time focusing on their career and traveling the world. It's likely to be the result of all three of these factors and more.
What is clear, however, is that this is not a theme unique to the United States. Death rates are surpassing birth rates in Europe (as this map shows), while countries like Japan are seeing their rates plummet – in 2017, fewer than 1 million births meant the entire population fell by more than 300,000.