Plastics didn't get introduced until around 1940's. It's true that population has exploded around 1950, but that could be due to a lot of other factors: better healthcare, better work conditions, reduced mortality rates, globalization, tech, awareness. Maybe bio-accumulation delay too? I don't know.

Most likely there are many different factors though, access to contraception, increased work load, women entering workforce, education, general preference not to have kids due to bleak outlook.

Microplastics has already been shown to decrease reproduction in animals so it's likely impacting humans too. In any case, prefer not to have it in my body ;(