Some ankle swelling is very common in the second half of pregnancy. If you develop pain and swelling in one calf, compared with the other, you should seek medical advice, as it may be the first sign of a leg clot (same problem as economy class syndrome). A clot in the leg is more likely to occur after birth.
There are a number of ways to assess the possibility of a DVT, D-dimers and blood flow (Doppler) assess of the legs or area causing concern. If a DVT is diagnosed an anticoagulant is used to reduce the risk of more clots and manage the current DVT.