While putting on the bulk of your pregnancy weight in the first trimester isn't the end of the world, it does cause some concern. If you started your pregnancy at a healthy weight, you should plan to gain 25 to 35 pounds total throughout your pregnancy. If you gain the majority of that early on (say, 20 pounds in the first 20 weeks), it may be hard to slow down your weight gain, and you'll end up packing on much more that what's recommended. Gaining too much weight during pregnancy can raise your risk for complications like gestational diabetes and preeclampsia, and makes it that much harder to get back in shape after your baby arrives.
Take a look at why you may have gained weight so quickly. Is your diet high in fat and calories? Are you eating a lot of sweets and fast food? It's fine to give in to some pregnancy cravings now and then, but now's the time to eat healthier than ever -- not to treat your kitchen like a 24/7 buffet. Keep in mind that you only need to consume about 300 extra calories a day while you're expecting (and less in the first trimester). That turns out to be a lot less than you'd think -- about the equivalent of a cup of cereal with milk and a banana.