It's a controversial subject with many varying opinions. Mine is that it's up to each individual. I would ride if I were pregnant but I'd probably stick to horses I deemed safer. I'd also continue working. Exercise is key and so is a happy mind.
I would stick to groundwork once I found out or at the very least before the 3rd month is up. There is so much that can go wrong even if you take every precaution. Why add any more risk? Why risk placental abruption, losing the baby, or worst (imo) causing the baby an injury, even a lifelong disability due to a fall? Even the most calm horse can have an off day, or you can get dizzy and fall off. Your body is not the same when pregnant as it is when you're not. We shouldn't treat it like a disease, but we also shouldn't ignore it because there's a developing human in there. Pregnant women have nothing to prove. There's no need to ride. Being on the ground with a horse imo is just as fulfilling as riding them imo. They're generally great to be around and will lift your spirit even if not on their backs. It's a personal preference yes, and I cannot tell anyone not to ride because it's not my baby, but I do feel it's an unnecessary risk, even if it's a tiny one. Most of us don't ride a horse to work or to get groceries or go to the doctor for an appointment. We do this as a hobby. So it can wait.