As explained above, if there is a discrepancy between how far along you are supposed to be and the size of your uterus on pelvic exam, this would be an excellent indication for an early ultrasound. If this pans out to a miscarriage scare, there are extra blood tests that can be done to check appropriate progress of your pregnancy. If everything between the dates and size jives and all else seems well, depending on your obstetrician, an initial early ultrasound might be done to demonstrate a normal-appearing fetus, to confirm the agreement with your date based on your last menstrual period, and to begin the actual bonding process. Except for a change in the due date, which may just be arithmetic, any other ultrasound surprises (like twins, for instance) will be your ticket out of the "normal" pregnancy group.
Many obstetricians feel comfortable getting two ultrasounds before 20 weeks, because the accuracy of the dating of the pregnancy falls off dramatically in later pregnancy. But because most babies grow at about the same rate before 20 weeks, this window is a good time to compare your baby with what's considered normal growth. The first ultrasound is often obtained at or before twelve weeks. The second one, if that is the usual practice of your doctor, will be obtained at least a month later so that an expected and appropriate interval growth can be documented. The use of ultrasound is a financially controversial matter. Insurance companies will point to studies that indicate that routine use of ultrasound will not change the pregnancy outcomes of the general population.
During the first trimester, the staple of each visit is the blood pressure, weight, and urine values for sugar (glucose) and protein. Weight becomes important when following a patient with morning sickness. Actual weight loss, which will prompt weekly or even daily visits instead of monthly, can result when morning sickness worsens into hyperemesis gravidarum. Unless you have chronic hypertension or kidney disease, you shouldn’t have blood pressure problems or protein spilling in the urine, which are more likely to be problems later in the third trimester.
Since the first trimester is the highest-risk miscarriage zone, any bleeding will prompt ultrasounds more often, sometimes even weekly. You'll be out of the normal group for this as well, and serial blood levels of the pregnancy hormone, hCG, will be drawn to watch for ensuing miscarriage, ectopic pregnancy, or resolution toward a normal pregnancy.
Fetal heart tones are difficult to hear during the first trimester; so hearing the heartbeat will have to wait until the second trimester.
The first trimester is important for assessing any history that may prove prophetic for the rest of your pregnancy, checking your blood and body for infections or diseases, watching your weight and the growth of your baby, and being alert to miscarriage scares. Since the second and third trimesters represent a continuum, they are dealt with here for the sake of giving you a global perspective - the big picture.