Hi M.~
I read the other response and to the best of my knowledge, there is no blood test that can be done. Clot in the legs are usually detected by ultra sound. I had DVT (Deep Vein Thrombosis) with my first pregnancy in 1994. Granted, I am overweight, and that is also a contributing factor, but the weight isn't the only reason for the clots. Tell your doctor you want an ultra sound on your legs to check for DVT. When mine was detected, it was explained to me that a certain amount of thickening of the blood is common but the thicker blood can also lead to clots, and they can be deadly. They usually start in the legs, and if they break off, they can go to the lung (called a Pulmonary Embolism, or PE) or to the brain. Both can be deadly.
Common treatment for blood clots is a blood thinner called Coumadin, however pregnant women shouldn't take it because it could harm the baby. They do have shots you could administer to yourself, and the one I was given was Heparin, but I heard of another, but don't know the name. I beg you to get yourself checked out. This nearly killed me twice, and I can no longer walk upstairs without having to sit down for 15 minutes to catch my breath. Don't ask your doctor, tell him you want to be checked out. If your current dr. says it isn't necessary, get a second opinion. This is serious condition and should to be treated as such.
I wish you the best of luck for the duration of your pregnancy and with your 3 yo...this is a wonderful time for you and I'm praying you are okay. Please let me know the outcome (my email address is ____@____.com) and again, good luck to you.
J.~