Congrats on your pregnancy.
If you want to use a permanent color, it is considered safe after the first trimester. Consider going a little longer between touch ups to limit the amount of chemicals you are putting on your head. Generally, women touch up their color about once each month. Consider waiting eight weeks between sessions. This cuts your exposure in half over the length of your pregnancy. Unless your hair grows very fast, you may not even notice the difference.
If you are still concerned about the safety, consider alternatives to coloring your whole head. Highlights are a safe alternative to hair color. These can be applied by the stylist pulling sections of your hair through a cap or with foil for longer hair. This can give you the look you want without the fear of exposure to chemicals. The color does not touch the scalp and is not absorbed into the bloodstream. With a little thought and planning, you can look beautiful during your pregnancy.