This is a common question. As with most mothers, you are obviously and justifiably concerned about the health and well being of your offspring. While severe trauma to the abdomen can be dangerous for the fetus, in general, a bumpy car ride for an otherwise healthy mother and fetus should not be a problem. Therefore, to answer your question, yes -- it is likely safe to ride in a mildly bumpy car while pregnant.
In general, your pregnancy goes through many stages. By five weeks, the fertilized egg and sperm union has already multiplied many times and is firmly connected to the inside of the uterus. Therefore, there is little concern of the bumps in the road "dislodging" the pregnancy. Direct physical trauma can cause problems, but the indirect full body motion of bumps should not be a problem.
That being said, the constant bumping of the road over years and years can be damaging to your spine. Therefore, many buses provide "shock absorbing" seats for their drivers. You may want to discuss this with your employer.
I would recommend discussing this with your primary OB/GYN. He or she can provide you with more information. There are many things to keep your pregnancy safe and he or she can help you with this.