Fewer, not less.
Two relevant numbers that frequently get conflated: Number of children per woman, and number of children per mother.
Family size (children per mother) relatively constant in developed countries for decades. If you account for much higher (50% or so) child mortality in pre-industrial times family size has declined over time but only a little.
Number of women having children, often called fertility rate, has declined dramatically in most of the world in the last 50 to 70 years because fewer women have any children at all.
Declines in either measure mean fewer children, but those declines likely have very different causes. People don't find partners to marry and have children with, for reasons that vary across cultures, but economic pressure looks like a major cause.
