Whether or not to ride when pregnant depends on a lot of different factors, the most important of which is your doctor's advice. Old wives' tales should be pretty far down the list, though they're unavoidable when pregnant.
For what it's worth, I rode until my 7th or 8th month, with my doctor's support. Here's what influenced that decision - I was very fit and healthy, and had a easy pregnancy, even though technically I was considered "high risk" as I was pregnant with my first child at 39. I was an ex professional and very experienced rider. I had a very safe and reliable horse. I just hacked lightly and trail rode, no jumping. I was advised not to get my heart rate above a certain level. Doctor also told me that during the first trimester, the risk in riding was primarily to the baby, but in the second and third trimester, the risk was mostly to me - the baby was well protected, but if I fell, I could sprain or tear an already stretched abdominal ligament and then have a painful delivery.
I stopped riding when my balance was so affected that I couldn't climb up on a fence to mount.
I know a lot of folks with similiar stories to mine; and I know a lot who stopped riding as soon as the pregnancy was confirmed. Neither is wrong, it's a highly personal decision.
Try searching this forum with "riding while pregnant" there are a bunch of threads on this subject and you'll get an even wider variety of opinions.