Sometimes, but not always, there may be signs that a miscarriage is pending. None of these things means a miscarriage is certain, but usually you will want additional monitoring if they happen.
Bleeding: Bleeding that starts and stops and starts and stops is often a sign that your hormone levels are falling. While you may still be okay, you need to have your blood hCG pregnancy hormone levels monitored. Heavy bleeding that soaks a pad in an hour is a sign that a miscarriage may be beginning. See the sections on "definite signs" for more information on what to do. Keep in mind that while bleeding is always scary, 70% of all pregnancies have bleeding at some point. Both of my normal pregnancies had bleeding, heavy and red. And the one I lost at 20 weeks never even spotted.
Cramping: You are going to feel a lot of random cramping down there the whole pregnancy. The only time cramping is a concern is if you are breathing in a labor-like huff, or if you also begin bleeding with the cramps.
Loss of pregnancy symptoms: This is a question I get all the time. While the complete and sudden loss of pregnancy symptoms can signal a pending miscarriage, usually it is not the first sign. You will have many days where you don't feel pregnant, when the nausea abates for a day or two, or your breasts are less sore. This is expected and not a concern at all. Around weeks 10 to 14, this is completely normal, as your hormone levels even out and the placenta takes over. The loss of pregnancy symptoms during a miscarriage is usually something you see in hindsight, not ahead of time.
A pregnancy test that is positive, then negative: This is a classic sign of an ectopic. Often you may also see spotting. If you have taken a pregnancy test that is positive, then another one a few days later that is negative, alert your doctor immediately. You want to rule out an ectopic or take care of it before you have to go the surgical route. If you are taking the tests in the same day, though, you might be right on the edge of a positive result, and urine later in the day may not be concentrated enough to keep the test positive. Test again the next morning to be sure.
You should always call your doctor when you are worried, however; because it is better to call for something that does not turn out to be a problem than to stay up half the night worrying about it.
If you are still positive you are having a miscarriage, check the section on definite signs of miscarriage.