According to WebMD, the first signs of labor include the baby dropping into the pelvis, the passing of the mucus plug, contractions, water breaking, and effacement and dilation of the cervix. Every pregnancy is different, and therefore not every symptom is necessarily present when a woman goes into labor.
Sometimes a woman feels what are called "practice contractions," or Braxton Hicks contractions. The American Pregnancy Association notes that these are different than true contractions because they do not happen consistently and do not follow a pattern or particular timing. They do not get progressively closer together, longer or stronger, and they usually stop when the woman changes position.