1 2nd-trimester fetal ultrasound is done to: 2  Estimate the age of the fetus (gestational age). 3  Look at the size and position of the fetus, placenta, and amniotic fluid. 4  Determine the position of the fetus, umbilical cord, and the placenta during a procedure, such as an amniocentesis or umbilical cord blood sampling.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.