-A curious adult from Illinois
Too much blood clotting in the mother can cause cystic hygromas
to develop in fetuses. The Factor V Leiden mutation you describe definitely leads to increased blood clotting. There are some reports that the MTHFR A1298C can also lead to increased blood clots, but this is much less well established.
Usually blood flows freely to carry nutrients and oxygen throughout the body. However, if an injury makes a blood vessel leak, a solid clot forms to plug up the hole. We've all seen this with a cut finger or a skinned knee. Clots can get dangerous, though, if they end up clogging up a blood vessel.
Clot formation is very tightly controlled to stop bleeding but not clog up blood flow. The process of clotting, called coagulation, requires many steps. If something goes wrong at any stage, there can be a problem.
For example, you can get too much clotting, like with Factor V Leiden. Or too little clotting like happens in hemophilia. The condition of unwanted clot formation is called thrombophilia.
How does Factor V Leiden affect clotting? A clot is a sturdy mesh of protein fibers called fibrin, woven around cell fragments called platelets. Fibrin assembles this mesh in the final step of a chain reaction that includes Factor V. The clot doesn't get too big because active clotting factors are quickly shut off again.
It works in a similar way to a runner in a relay race who sets off at full speed when he receives the baton. But, he stops running once the baton is passed further along. Factor V Leiden is an abnormal form of Factor V that stays active when it should shut off.
Do many people have Factor V Leiden? This mutation is the most common cause of inherited thrombophilia. A single copy of Factor V Leiden is found in about 5%-7% of the Caucasian population in the U.S., especially in people of Northern European ancestry. (Remember, most genes come as 2 copies, one from your father and one from your mother).
A person with one good Factor V gene and the other a Factor V Leiden has a 4-7 times increased risk of clotting. Having two copies of Factor V Leiden is uncommon, and more serious. The risk of clotting is increased 80 times over normal. This occurs only in 0.06-0.25% of the population.
Factor V Leiden increases blood clots in veins, especially in the legs (called deep vein thrombosis, or DVT). The most serious danger arises when a large piece of a clot breaks off and travels to the lungs (a pulmonary embolus). This can make it hard to breathe.
It may sound very scary to have this mutation. But actually, many people with Factor V Leiden will never notice any unwanted clots, and will never know they have this abnormal protein. The mutation is only discovered if other factors that also increase clotting are present. In your case this was pregnancy.
During pregnancy, blood clots form more easily. The presence of the Factor V Leiden and, possibly, MTHFR A1298C mutations combined with pregnancy makes the tendency to form clots much greater.
The other mutation you mention, MTHFR A1298C, might indirectly affect coagulation. MTHFR is a protein that helps along an important chemical reaction. The A1298C version of the protein does not work very well. This can lead to an increased amount of the chemical homocysteine in blood, and it's excess homocysteine that increases clotting.
As you can see, the tendency for blood to clot is affected by many factors. This is similar to traits like body weight and blood pressure, which are influenced by multiple genes, environment and behavior. Unwanted clotting can often be reduced by being active and staying well hydrated. Changing your diet can lower Homocysteine levels.
But sometimes environment and behavior can't be modified or don't reduce the medical risk enough. In that case doctors use drug treatments. Heparin is one kind of medicine that decreases blood clotting.