Unfortunately, miscarriage can be a time-consuming event, and most couples wish to get it over with once they know for sure a pregnancy won't succeed; but, your doctor will want to know such a thing with absolute certainty, because she wouldn't want to intervene in a normal pregnancy. If there really is a cause for concern, your doctor is already on it. While looking for all other innocent reasons to explain away bleeding, ultrasound and blood work are used to find answers as soon as possible.
If seeing is believing, then an ultrasound can be an epiphany. There's nothing like seeing a normal fetal impression with a good fetal heart motion to reassure you when there are even the most troubling symptoms. And besides documenting the health of the fetus, ultrasound can put another worry to bed—ectopic pregnancy. Seeing the pregnancy within the uterus itself will rule out a pregnancy in a tube or anywhere else, except in the rare occurrence of twins—one in the uterus and one in the tube. And if there is an ongoing concern, like a subchorionic hemorrhage, serial ultrasounds weekly can watch a blood clot shrink away.
Made by the placenta, hCG rises and accumulates, only to plateau around the end of the first trimester. By that time its levels can reach the hundreds of thousands, but in early pregnancy the numbers are much smaller. This makes it possible to quantitate interval increases in this hormone. In early pregnancy, the amount of hCG should double every two days or so. For example, an hCG of 1800 should rise to about 3500 to 4000 over 48 hours. If it doesn't, but it's close, then another repeat sample two days after that will be helpful. If it does in fact double, then this is a good indication that everything is going fine. If it exceeds the doubling tendency, twins or molar pregnancy (see below) may be brewing. Progesterone is maintained at high levels throughout the pregnancy. Although a pregnancy can be in trouble with low progesterone, most of the time the opposite is true: the progesterone is low because of a faulty pregnancy. There are absolute values that give obstetricians comfort (15-20 ng/ml). If the progesterone were to come back low, this could indicate a problem even if the hCG is reassuring (usually the hCG will not be reassuring with an abnormally low progesterone). Borderline low may warrant progesterone supplementation, but really low values can be assumed to be because of the likelihood of miscarriage.
Your Rh-Factor is a necessary item of information that was probably obtained with your initial blood work. If you're Rh-Negative, you will need a RhoGam injection with any bleeding episode to prevent your blood from making antibodies to the baby's blood. This assumes that any bleeding involves mixing of the two to some extent. Studies of the immune system, anti-nuclear antibody (ANA), antiphosolipid, lupus prep, etc., are usually reserved for women who have had recurrent miscarriages.
Usually a threatened miscarriage will commit itself within days of the first bleeding. Some do linger, but eventually the truth will become evident. Either the cervix will open, making it an inevitable AB; tissue will pass, converting it to an incomplete AB; or the entire gestational sac will pass, as a complete AB. Or it will go on to be a normal pregnancy. There's no such thing as an incomplete miscarriage that goes on to term.
Patients sometimes fear that if they get over the hump of a threatened miscarriage, what's left will result in a terribly abnormal baby at the end. This simply does not happen. Miscarriage is an all-or-none event. It'll either commit to miscarriage or commit to a normal pregnancy (barring any unrelated problems).