Yeah, we were ignorant.
The ovum wasn't discovered until mid-nineteenth century, rhythm in the twentieth, sympto-thermal method later in the twentieth.
Some women might have noticed that they conceived early in the cycle, when they were in the mood and the secretions were clear, and not when they were close to the period and not in the mood, but it wasn't generalized or publicized.
And women had fewer periods back then - nursing tends to stop them, and when the cycle starts up again - they're likely to be pregnant quickly. So fewer opportunities for observation there - but more opportunities for observation with more pregnancies, I guess.
Maybe when the human race suffered from dire poverty and early death and primitive medicine, we needed to have more babies, so God didn't let us find out about the cycle until later...but I am suspicious of interpreting design from every fact.
Mrs VS
My wife and I thought that breast feeding was an effective form of natural birth control. To our surprise, and joy, we are expecting our third child in August. We couldn't be happier. Once again abstinence is proved to be the only 100% effective method.