I actually have the unfortunate circumstance of knowing quite a bit about this
The ancient Romans used coitus interruptus, olive oil on the you-know-what, and then some weird, weird stuff:
there were magic talismans to ward off pregnancy, there were various rites that would do likewise, and I think there was one that involved jumping up and down.
If there was a pregnancy, the most common form of abortion was to take a drug that would be deadly enough to kill the kid without (ideally) killing the mother.
If a child was born, the father had the power within the first two weeks to kill it, no questions asked. He would frequently use this against his female children.
Even if a kid survived all this, he had a 33% chance of death before hitting age one because of disease, famine, and the like.
Oh, I should mention that medical practice was all screwed up: pregnant women were supposed to be active until they gave birth, and then to be active immediately thereafter. Babies were to be given goat's milk and honey rather than mothers' milk for the first few days because the mothers' milk was chunky (truth: full of proteins and antibodies) and therefore "bad"
The real reason women didn't become pregnant was by lactating for a year or more after childbirth. When you're lactating, you're not fertile (usually). The average, btw, was I think six children per fertile woman.
Later, many of the more barbarous practices fell off, and the most common forms of contraception was coitus interruptus.