The simple fact that a woman's growing uterus puts veins and internal organs under pressure caused many of the symptoms that pregnant women frequently meet with. Fluid retention, and thereby swollen ankles too, is no exception here. This increased pressure causes blood flow from your legs back to your heart to slow down somewhat. And blood remaining in the legs for longer causes some of its fluids to escape to other tissues, causing them to swell. Not only is the speed of blood flow reduced, the very chemistry of blood also differs while you are expecting. Ankles are a common target for this symptom, which is also called edema, or oedema if you are British :).
But the entire lower leg can be affected, and hands and wrists also pool fluids on occasion. The further along in pregnancy you are, the more likely you are to experience fluid retention. Indeed, edema is mostly a third trimester "problem". Some people also report that fluid retention happens more often in warmer climates, and during the summer period. Do you need to call your doctor about fluid retention? Probably not, unless your hands are extremely swollen, and you notice swelling in your face.
Swollen ankles shouldn't really concern you, but there are some things you can do to minimize the swelling if you find it irritating. Try moving around as much as you can, and not staying in any one position for too long. Staying off your feet is also said to reduce swollen ankles, because it allows for the best possible blood flow to your lower extremities. Eating well, and drinking plenty of water (believe it or not!) helps keep the swelling down, and wearing loose and comfortable clothes also aids the blood flow. The ultimate and guaranteed cure for swollen ankles is giving birth!