Why do I have protein in my urine while pregnant?

Several reasons. Protein in the urine during pregnancy can both be normal; due to increases in blood flow to the kidneys or due to dehydration, or abnormal; as in the woman developing preeclampsia or other renal diseases. Usually, normal proteinuria is either trace or 1+ on a urine dipstick and is not associated with high blood pressure or swelling in the hands and face.
Protein in the urine. Protein in the urine or proteinuria is due to leaky kidney tubules, and is commonly seen in pregnant women. High loss of protein in the urine measured over 24 hrs. Can be a diagnostic of preeclampsia, and should be followed up with your obstetrician. The initial studies from the uk on proteinuria and uric acid clearance have documented this condition, and discussed the associated risks.
Leaky vessels. Your blood volume is markedly increased in pregnancy and that extra volume stretches all of your blood vessels so they leak. The leaked fluid is serum which is protein. The leaked serum in your kidneys comes out in your urine. This is worse for hypertensives, and one of the early signs of pregnancy induced hypertension. That's why your doctor will ask you for urine samples.
Caution. Protein in the urine while pregnant can signify renal disease. The usual explanation is the start of preeclampsia or toxemia of pregnancy. Follow up with your doctor as soon as possible.
High blood flow. The reason that many pregnant women have a small amount of protein in their urine is related to high blood flow through the kidney. If there is more than a minimal amount of protein in the urine it can be due to kidney disease or pre-eclampsia. That's why we check your urine every visit.
Leaky kidneys. All pregnant women have leaky kidneys while pregnant and typically can have some small amount of protein and glucose in the urine.
May be a sign . Women may have protein in the urine for many different types of reasons. Some include a urine infection. Some women have underlying medical condition such as high blood pressure, diabetes or lupus that causes this. In pregnancy women can develop pregnancy induced high blood pressure and this often accompanies protein in the urine. So this should definitely be followed by a physician if it develops.