Birthmarks are generally congenital skin abnormalities manifested at birth. Most birthmarks do not affect the normal growth and development of children, nor do they affect the function of the local skin. Birthmarks can be detected only after the baby is born. The birthmark cannot be detected by ultrasound while the fetus is inside the mother’s body, even with 4D imaging. The items examined by 4D ultrasound are one of the prenatal screening programs for fetal malformations, while birthmarks do not belong to the category of malformations, and even if the fetus has birthmarks, they cannot be detected by 4D ultrasound. In general, 4D ultrasound examination is only done during specific weeks of pregnancy, usually between 20 and 24 weeks of pregnancy, because the anatomical structure of fetal tissues and organs in this period has been perfected and can be displayed by ultrasound; moreover, the space in the uterine cavity as well as the volume of amniotic fluid and the thickness of the placenta are also suitable for 4D ultrasound examination. The accuracy of screening for fetal malformations can be affected by too early or too late examination. In addition, due to the influence of thick subcutaneous fat in the mother’s abdomen and factors such as too little amniotic fluid or fetal position, the ultrasound cannot display some parts of the tissue structure well, so the detection rate of 4D ultrasound for fetal abnormalities is not 100%. In short, fetal birthmarks cannot be detected by color ultrasound or 4D ultrasound.