The study can be performed transvaginally (with a vaginal probe) or transabdominally (through the abdomen). During the first trimester, doctors usually use a transvaginal ultrasound rather than an abdominal ultrasound to obtain information about the pregnancy. Transvaginal ultrasound provides the most accurate information in the early stages of pregnancy due to the fact that by this time the ovum and fetal pole are still extremely small, and the vaginal probe allows you to get as close as possible to a developing pregnancy. In a transvaginal ultrasound, the doctor inserts a special thin probe into the vagina to take a series of measurements, including the size of the ovum, the size of the yolk sac, the coccyx-parietal size (CTE), and the presence of a heartbeat.
On a transabdominal ultrasound, a woman will be asked to come in for a scan with a full bladder because this positions the uterus in a way that makes it easier to take measurements. The doctor then applies the gel to the lower abdomen and uses a transducer to measure from different angles.