You can work out while you're pregnant, but there are a couple small guidelines to follow so that you don't birth a little cripple. I'm not making some sort of statement against handicaps, I'm just assuming most mothers don't want to kick out a quadriplegic, blind kid from their birth canal. "Yay! My child will never grasp a language!" I gather you understand why following these guidelines could be an important consideration.
Okay, first off, you can't actually lose
body fat, but you can put on some muscle and improve cardiovascular, respiratory, and aerobic capacities during that time. You can lose body fat just fine when it's over- and I'm sure you'll still be motivated. But if you start trying to shed the pounds at any time pre-discharging the little fetus-item, it's probably a bad idea (at best).
Additionally, if you're pregnant now, and you weren't working out before you were pregnant, feel free to start- but don't be crazy about it. Going overboard on an unaccustomed body isn't going to help you much when your system doesn't respond right and the child is engorged with heat. Once your core body temperature hits 101, it has the potential to harm the fetus- and it's easier to get your core body temp climging while pregnant. The vast majority of all the calories you eat are always released as heat (entropy). There's no way around that, it's just how you're built. So when you increase your caloric intake (i.e. pregnancy should do that a little) you increase your heat. Then coupled with exercise that requires greater blood flow and energy expenditure (i.e. working out while you're pregnant), you get even more heat. If your body is not already used to handling this (through negative feedback responses), it won't really all that well. So you end up with this burned up little baby.
Furthermore, an argument exists that the uterus requires a certain amount of blood flow to support the baby- and you have a finite amount of blood. At rest, your muscle is using about 15% of it. During exercise, this number shoots up to as much as 85%, leaving 15% for the entire rest of your body- your digestive system, your brain, kidneys, and so on, including the uterus. Now, realistically, you still have no reason to worry. The human body will almost never take blood away from the placenta. If a mother's body wasn't all sorts of skilled at maintaining proper blood flow there, the entire world would be crippled. However, it can take blood away from you. So there's a bigger chance that you'll end up hurting yourself with this one, and that kind of opposes the purpose of exercising to become healthier. But regarding blood detraction and the health of your baby, if you've never worked out before, and you go completely overboard, your body might not know how to handle the additional stresses that it hasn't routinely been subject to before. By that time the heat's doing it, but the possibility of blood detraction, however unlikely, still is a conceivable issue. And realistically, you may not regret it when it's all over, but your crippled fetus might. It's just that it'll never be able to speak, so it won't have the privilege of telling you. That and it'll be dead in a week, so... It's up to you.
The dead-fetus stuff is of course mild, kind of disgusting exaggeration- but honestly, I wouldn't chance it just so that your maternity dress is less snug around your hips. The temporary status of your hips is not that critical compared to the entire life of a human being. So just pace yourself. I know you're ultra motivated but it's not that hard. If you hit pregnancy and have to work-out like a fiend to the point where your internal temp is climbing that fast, consider that you probably have psychological problems. Fix that during your pregnancy. A psychiatrist will in no way destroy your child like you probably will otherwise.
Regarding pacing yourself: if you've been working out before becoming pregnant, use that exact same intensity. If you weren't working out before, be reasonable. Ease into the workouts. Basically just exercise healthfully as if you're not pregnant, providing you've graduated from the training techniques of the 80's, i.e. Rocky-style-workouts where he does crunches shirtless while some old man slaps his stomach a lot. Don't do that. Even if you're not pregnant. But if your exercises are legitimate, just train normally.
Once you hit the second trimester, tone it down some. The literature suggests about a 30% reduction in intensity, but a) that seems a bit much, and b) it's ridiculous being as nobody is going to calculate their intensity. But seriously, just tone it down a little bit. If you feel like doing the math, feel free- but that means you're a huge nerd who's about to birth a Star Wars entrepreneur, which is fine. But it's also fine to just barely ease up a little in transition from the first to second trimesters. Then do the same thing when you hit the third. And by the time you're in the third, you shouldn't be doing any exercises that cause you to lay on your back. The weight of your little Star Wars child is getting hefty enough to cause problems in the supine position. So figure out another way to work your abs and don't do bench press.
And you're going to have a bunch of laxity in your joints- especially as the pregnancy progresses. Don't confuse this with flexibility. If you're stretching or doing full range of motion exercise, ease up a bit. Otherwise, you'll have sweet ass joint problems post-pregnancy.
As far as eating is related to exercise, always have a snack or a meal beforehand, and make sure you get plenty of water. And lastly, always see a physician about anything unusual. Actually, see Fred Frank. He's this little Jew OBGYN. Easily the best one in the northwest. He says he's not, but factually, he is. He'll hook you up. Tell him Courtney says hello- and that he's going to be devastated in the next Chess game.
After you're done with the whole pregnancy thing, you can do the losing body fat thing. Actually just do whatever you want.