Spotting during pregnancy, although it may spook many new moms, is a normal part of pregnancy. According to a study done by Columbia Presbyterian hospital, spotting occurs in about 30 percent of women within their first trimester. This is often caused by implantation, the process where the fertilized egg attaches to the uterine lining. 
Bleeding due to implantation can often occur before a woman knows she is pregnant, or it can occur afterwards. However, implantation can happen anywhere from 6-12 days after possible conception. The spotting is usually lighter than the flow of bleeding found in menstrual cycles.
Cervical polyp is another potential cause of spotting in the first two trimesters of pregnancy, this occurs due to a harmless growth on the cervix that can appear due to higher levels of estrogen.
During pregnancy, there is an increase in the number of blood vessels around the cervix. Contact with those blood vessels through sexual intercourse or a procedure such as a gynecological exam can result in light bleeding as well. Light bleeding after sexual intercourse or a pap smear is considered normal, and should not be cause for alarm regarding miscarriage or other pregnancy complications.
However, if bleeding is heavy or constant, a physician must be called to ensure that there are no problems with the pregnancy, especially if heavier bleeding occurs in the second or third trimester. The color of the discharge should not be alarming, as it can vary from lighter shades of red to brown.
The key indicator to look for is the amount of blood and the frequency of spotting. In order to monitor the vaginal spotting, a pad or panty liner must be worn in order to observe the spotting and ensure that it is not caused by a serious complication.
Although spotting can be alarming, it is again not an instant sign of miscarriage or complications. Symptoms of miscarriage include a heavier flow of vaginal bleeding, cramping pain felt low in the stomach (such as what is often experienced in a woman’s period), and tissue passing through the vagina. Symptoms of ectopic pregnancies (pregnancies implanted somewhere other than the uterus) include that of miscarriage with more abdominal pain and low levels of HCG.
If spotting or light bleeding occurs in the third trimester of pregnancy, this can be the indicator of more serious problems. Vaginal bleeding can be an early indicator of preterm labor; the bleeding associated with preterm labor can begin up to a few weeks before contractions actually begin. Preterm labor spotting can also be accompanied by vaginal discharge, a low and dull backache, stomach cramps, and diarrhea. A physician should be consulted if any bleeding occurs during the third trimester, as it could be a symptom of preterm labor.
Spotting during pregnancy can be a sign of several different conditions for a pregnant woman; the key factors to observe are when the bleeding occurs during pregnancy, and how severe the spotting is. If the spotting is heavy and continues over the course of more than a day, it is a sign of a potential complication and should receive some degree of medical attention. If it occurs early in the pregnancy and is light with minimal pain, that is considered to be a regular part of the pregnancy process.