On the contrary, "trying for kids" was accompanied by an elaborate ceremony to kick off the process. This big ceremony was often so successful that the kids would arrive 5 months after the ceremony!
Trying for kids means making sure you have sex at the right times of the month, something humanity has known about for millennia. Equally people not 'trying for kids' made sure to only have sex at the 'wrong' time of the month, again something humans have been doing for millennia.
Up until very recently, there was very little knowledge that pregnancy lasted nine months... even among the 'learned'. Old pregnancy manuals talk about pregnancies taking anywhere from a few months to many many months. Without ultrasound and HcG tests, there is very little indication a woman is pregant until she starts showing or feeling the baby (and even then, a new mother may not even notice until she's showing anyway).
As for fertility awareness, I don't believe the methods used prior to the modern day were super effective. Regardless, the knowledge was not present.
So I'll restate my claim. Up until the modern day, there was no such thing as trying for children. There were married couples engaging in socially sanctioned sex, which obviously leads to children. Or there were fornicators that we knew could have children, but was socially unacceptable, and few fornicators actually wanted children from their unions.
If a woman's periods are regular and she experiences no bleeding in pregnancy. Many women's periods are not regularly, especially so if many woman are malnourished. Moreover, bleeding in pregnancy (esp early pregnancy) is fairly common. Thus, if you read older gynecological manuals, they clearly claim that pregnancy lasts some range of months. Most people realized that it was centered around 10 months, but there were many doctors claiming that the ranges were much higher than we'd accept today, because they had no reliable method to test for conception.
If you bleed like some women do during pregnancy, then you may have no indication you're pregnant until you obviously show. It's unlikely, but not impossible, and the doctors had no objective measure by which to say otherwise unless they could feel or hear the baby.