Is horse riding safe in the first trimester of pregnancy?

Queen Elizabeth's granddaughter, Zara Philiips, is a famous equestrian and she was criticised heavily because she continued riding during her pregnancy. She was pictured playing a game of polo when she was 3 months pregnant and set tongues wagging. But she gave birth safely and her baby is healthy. My opinion is that because Zara Phillips is a professional athlete, her body is used to the movements that come along with horse riding and she is aware of the safety aspects of the sport. If you've never ridden a horse and want to try, best to do it after pregnancy. Like what Hui Qun said, you can fall off the horse and risk placental abruption.