It seems odd they want to restrict your activities. Active pregnant women have a reduced preeclampsia rate (which rises as you age and is high for 1st preg). Mild to moderate exercise also helps reduce the risk of gestational diabetes. Hmmm, without a cause for your prior preterm, I would think they would be cautious at first, ride but carefully, light, not hard. As you progress, you should watch for signs that the exercise is not working for you....bleeding or spotting, loss of amniotic fluid which is easily noticed and smells like the ocean (sorry), dropping/open cervix at exams. FYI: I had preterm at 24 weeks which resulted in a stillbirth. I was told NEVER have another child since I have a common but detrimental clotting disease. Well, I fell into the 15% fail rate with a condom (I also can not take hormones because of the clotting). They started me on heparin and I delivered (by induction) a healthy baby boy at 38 weeks. Every pregnancy is different. Just go easy and try to keep HR no higher than 130 in 1st tri, 120 in 2nd tri, and 100 in 3rd tri. This was what my OB told me. I found that 3rd tri activity levels were higher when I exercised. I guess my son liked it too!