Fetal ultrasound is a test used during pregnancy that creates an image of the fetus in the mother's uterus, or womb. During a fetal ultrasound, various parts of the baby, such as the heart, head, and spine, are identified and measured.etal ultrasound is a test used during pregnancy that creates an image of the fetus in the mother's uterus, or womb. During a fetal ultrasound, various parts of the baby, such as the heart, head, and spine, are identified and measured.