Swollen ankles are a logical consequence of the fact that you have more blood pumping around your body while you are expecting a baby, and that your kidneys have more fluids to process. In addition, veins are put under more pressure as your uterus grows and your internal organs get squished in the abdominal cave. Legs, and ankles in particular, are a common target for water retention because the blood takes more time to return to your heart from your extremities and fluids can build up in these areas. Swollen ankles can be annoying, but should be painless. If you are experiencing swollen ankles, try going for regular short walks, generally staying active and drinking plenty of water.
You should not wait until you are thirsty to drink. Lying down for a while if your ankles get really swollen is another possible remedy for this nasty but typical pregnancy condition. Edema can start in the second trimester (which is why we listed this symptom in the second trimester on our pregnancy signs page), but gets more common as your pregnancy progresses into the third trimester. Swollen ankles should not normally be a cause for worry, and there is no need to make an extra prenatal appointment if you have "fallen victim" to water retention. You should mention it at your regular appointment though.