I am always fielding questions from pregnant women about what they can do to stay active. Most of them fit into two extremes: They are either curious about prenatal yoga or they want to know if they'll be able to run an 8k when they're eight months along.
As a fitness coach, the first thing I ask them is what they were doing to stay active during the six months prior to their pregnancies. Unless her doctor specifies danger to herself or the baby, a woman can and should maintain the same course of physical activity that she did during the six months prior to pregnancy. Sorry, but horseback riding and downhill skiing would be exceptions to that rule, and maybe also fixed-gear bike racing and any other activity that may involve falling...
This "six month" standard has two very important applications. First, if you are already pregnant, it is NOT the time to start a new routine beyond a very structured prenatal class and light neighborhood walks. On the flipside, the second application is that if you are not already pregnant you can plan for the future. The better shape you get yourself into before you get pregnant, the better shape you can stay in while pregnant.
I will be posting a second article entitled "Fit -2- Labor" but let's get you into a healthy, fit pregnancy first. Start by thinking about your goals. What do you want to be able to do while you are pregnant? If you want to be able to lift weights, you need to get a routine that you don't mind doing for the next (9+6) 15 months! If you are a lifelong runner, you'll want to pick up the pace during the six months before you start trying to conceive.
You need to plan now for the modifications you'll have to make later. I said you could keep going; I didn't say you'd be going as fast. So build momentum now. Keep swimming, keep race-walking, keep going to aerobics classes. That momentum will carry you smoothly into the early weeks of pregnancy.
Of course, nausea and lack of energy during the first trimester will goof with your goals. But you will be so much more likely to stick with that planned activity if you already have the stamina and the strength for it. If you did it five times a week before you conceived, you should surely be able to do it three or four times a week during pregnancy, allowing yourself some down days.
Your baby and you will both benefit from your fit pregnancy. Studies show that women who exercise deliver larger babies with healthier lungs and hearts. You will have fewer aches and pains during pregnancy, and likely a shorter labor. The best part is that you will bounce back faster after delivery. You will still need those six weeks to recover post-partum, but your muscles will remember and you'll fall back into your old pattern much easier.
Your body will change drastically while you carry your child. You will gain weight, and your center of gravity will shift forward. Your ligaments will loosen, and your breasts will need a tighter sports bra. But you will be able to keep doing what you love if you plan for it.