As with most pregnancy aches and pains, you have your hormones to thank for this discomfort. During pregnancy, your body makes more of a hormone called relaxin, which keeps your ligaments loose so your baby can pass through your pelvis more easily. This relaxin surge begins shortly after you conceive, peaks in the second trimester, and remains steady until a few days after you deliver. As a result of this loosening, your pelvic joints (like your sacroiliac joint, which connects your pelvis and your back) begin to shift, triggering a dull, achy pain around the hips. Many everyday activities -- like sitting at your desk, getting in and out of the car to run errands, or caring for children -- can also strain these joints further.
To ease your joint pain, try placing an ice pack on the area several times a day or taking a pain reliever like Tylenol. (Don't take pain relievers like ibuprofen or aspirin, which are unsafe during pregnancy.) Daily stretches can also soothe your achy hips. Here's one that's especially relieving for moms-to-be: Lie on your back with your knees bent and your feet flat on the floor. Place a rubber ball between your knees and gently squeeze it. Do three times and hold for five seconds each. (Check with your doctor to be sure it's safe for you to lie on your back briefly.) You should also hang up your high heels, which mess with your body's alignment, and stick to wearing flats.
Another possible explanation could be sciatica, though true sciatica in pregnancy is really actually sacroiliac joint pain. In this case, your growing baby may be pressing on your sciatic nerve (located in the lower part of your spine). Women with sciatica usually feel pain around their hips or deep within their buttocks, and it often radiates down one leg or the other, depending which side your baby's leaning toward. Some women also report numbness or tingling in one of their legs. Parked at a desk all day? Stretch and get up to walk around a few minutes every hour. Swimming is also an excellent exercise for women with sciatica, because floating alleviates all that pressure on the sciatic nerve (just check with your doctor to make sure it's okay for you to exercise).
Although hip pain is very normal during pregnancy and usually nothing to worry about, if it becomes severe or debilitating, ask your doctor to refer you to a physical therapist. Because it is often the result of pelvic misalignment from the looseness in the pelvis, sciatica can be greatly improved by a physical therapist, who can teach you exercises to realign the pelvis.