[somenameforme]

Birth control dates back at least 3800 years, and probably much longer, as that 3800 year old reference comes from documentation accurately referring to various substances with spermicidal characteristics. [1] The same is true of abortion and ways to induce it.

And anybody who's had a child can tell you that getting pregnant is not as easy as you might think. In general women are only fertile for a window of several days per month, which they are capable of also determining due to various physiological changes that happen during that window (and also the fact that the window occurs during the, more or less, exact same time each month following their period). And even if you nail that window, the chances of a successful pregnancy are relatively low - only about 20% per month for young couples, and then rapidly decreasing for women beyond the age of 30.

Notably, in the century prior to the Roman Empire's collapse, fertility rates collapsed for reasons that are still unclear. Anyhow, this is all a very long-winded way of saying that the higher birth rates of the past weren't simply because of unplanned pregnancies. If they wanted to lower their fertility rates, they would have been fully capable of doing so.

[1] - https://en.wikipedia.org/wiki/History_of_birth_control