Sadly, somewhere between 1 in every 3 to 5 confirmed pregnancies ends in miscarriage. Miscarriage symptoms before diagnosis usually include vaginal bleeding and cramping low in the abdomen or back, but these signs don't always mean miscarriage (and not every miscarriage will have symptoms at the time of diagnosis). Any of the following signs of miscarriage is definite cause for concern, but only the first two are fairly absolute.
If your hCG levels are decreasing over time, you are almost certain to be having a miscarriage; this is among the most reliable indicators of an early miscarriage. In order to check your hCG levels, your doctor will probably request that you have two blood tests performed over a period of two days.
In many cases, there will be miscarriage symptoms before an early ultrasound is performed, but in a missed miscarriage or blighted ovum, discouraging ultrasound results may be the first sign that anything is wrong. Ultrasound can reliably diagnose a miscarriage in many cases. In other cases, a single ultrasound may not be enough to indicate a miscarriage, especially if the dating of the pregnancy had not been known prior to the ultrasound.
Vaginal bleeding in early pregnancy is the most common miscarriage symptom that people have before diagnosis. It's important to remember, too, that vaginal bleeding does not always mean miscarriage, so the best thing to do is see a doctor. Although your doctor won't be able to stop you from miscarrying if that is what is happening, your doctor will be able to perform tests to confirm that it is a miscarriage and to make sure your health isn't in danger.
Painful cramping also does not always mean miscarriage. In some cases, painful cramping can even occur in normal pregnancies. Whenever the pain is severe, however, you should always see a doctor right away to make sure that you don't have an ectopic pregnancy.
Most of the time, heavy bleeding and passing tissue during pregnancy is going to mean miscarriage, but it is still best to go to the doctor and have the tests to make sure. Remember that in early pregnancy, any tissue from the pregnancy is still going to be quite small and is not going to account for large golf-ball sized clumps that you may be passing. There are certain types of cysts and other conditions that may lead to passing tissue.