It's not uncommon for people to feel hungry all the time in pregnancy. The first trimester is an odd combination of feeling ravenous and nauseated, all at the same time. The second trimester usually brings welcome relief from morning sickness, but it can also be a time of insatiable hunger.
This is because the unborn baby needs nutrients to create bone, muscle, and other tissues. Eating a healthy snack every three hours or so should help avoiding overdoing it at mealtimes, and it will also help keep blood sugar on an even keel. Drinking enough water and eating high-fiber, high-water-content fruit such as apples, oranges, berries, pears, and plums will also help one feel full.