In the first trimester, doctors usually use a transvaginal rather than abdominal ultrasound to gather information about the pregnancy. The transvaginal ultrasound provides the most accurate information in early pregnancy, given that the early developing gestational sac and fetal pole are extremely tiny at this point and a vaginal ultrasound can get closer to the developing pregnancy.
In a transvaginal ultrasound, the doctor or technician inserts a thin probe into the vagina in order to take a series of measurements, including the size of the gestational sac, the size of the yolk sac, the length of the fetal pole, and the heart rate.
In an abdominal ultrasound, the woman will be asked to come to the scan with a full bladder, because this positions the uterus in a way that makes it easier to obtain measurements. The doctor or technician then spreads a gel over the lower abdomen and uses a transceiver to take measurements from a number of angles.