Transvaginal ultrasound is generally done early in a pregnancy to determine fetal age or to detect a suspected ectopic pregnancy. It is occasionally done late in pregnancy to determine the location of the placenta or in a high-risk pregnancy to monitor the length of the cervix.hy It Is Done. Fetal ultrasound is done to learn about the health of the fetus. Different information is gained at different times (trimesters) during your pregnancy. 1  1st-trimester fetal ultrasound is done to: 2  Determine how your pregnancy is progressing. 3  Find out if you are pregnant with more than 1 fetus.