N.S.
I'd say don't bother signing up. For sure, talk to your doctor, but you will find most doctors will be very conservative in their recommendations, and will more than likely suggest that after the first 6-8 weeks of pregnancy you should limit physical activity to a specialized fitness class designed for pregnant women, or low impact activities like swimming or a walking program. No pregnant woman should ever do any exercises that would require them to lay on their back like sit-ups etc.(there's risk of rupture and blood flow issues to the baby). Most doctors will advise against running, and any "high impact" exercises or "contact sports" after the 1st trimester for risk of rupture or premature delivery.
Another thing to consider is that, like the elderly and the very young, pregnant women are at a greater risk for heat distress and heat stroke in the summer. Especially on those really hot days in the 90's or higher. When you are pregnant, your core body temperature is already slightly warmer than normal to accommodate the fetus. A rise in your core temperature can sometimes not be safe for the baby. It is for this reason that most doctors will recommend against using a hot tub, or taking extremely hot bath or showers during pregnancy. Doctors will also recommend that when you get a high fever during pregnancy due to a cold, the flu or anyother illness you should contact your doctor immediately. Therefore, during summer months You need to be very mindful of keeping your core temperatures stabilize. Pregnant women need to drink more water than normal because blood volume increases to accomodate the baby. Some pregnant women with autoimmune disorders like Lupus should avoid the sun altogether.
Last but not least, there is the unpredictible. While no two pregnancies are alike, if you had a previous pregnancy that was "challenging" most often the next follows suit. If this is a first pregnancy, you have no idea what it will be like. Some people don't do well with nausea, some have sciatic and hip related problems that make walking let alone running difficult.
Also consider, that while you will for sure be doing everything you can to keep yourself and the baby safe, you can't control a soft-ball moving at high speeds. One impact could be enough to endanger or terminate a pregnancy. There's the risk of someone slamming into you, or worse you could fall on your stomach or on the ground, all which could prove dangerous to the fetus.
I'm probably sounding like Ms. Doom and Gloom, but pregnancy, though most of us liberated women would love to think is not an obstacle, is a time when we should be more mindful or our health, and the health of our newest addition.
I made the mistake of pushing the limits during my last pregnancy and wound up breaking my pelvis a month out from delivery. They can not give you a cast for this, and x-rays are dangerous for the unborn (causes birth defects), all the painkillers that could make a difference were off the list because of risk to the baby. So I had to lay in bed for two weeks in excruciating pain until it was safe to have a c-section. I couldn't bathe, I could go to the bathroom by myself, I couldn't do anything. Worse we didn't know if the baby was hurt. Ultrasounds can only tell you so much. Even with a saddle-block delivery was an experience in itself. IN the end, we all turned out okay. But in retrospect, not altering my lifestyle for the sake of the baby was a mistake. In short, while there are activities that you hate to give up for the birth of a little one, nothing is worth injury or worse. It's temporary, and there is always next year. Besides, if you're not ready to give up something, maybe you're not ready for a little one. Things are going to change, and you need to be ready for that. Remember, not all change is bad.
And while there is technology to help aid premature births, realistically, most babies born before 21 weeks have a high death rate. It doesn't take much to harm a baby in utero.
I myself took it for granted that women are tough enough to handle the day to day and a little extra physical activity on the side.