Fetal birthmarks usually tend to form during the 4th to 6th month of pregnancy, but the specifics vary from person to person. Many fetuses develop birthmarks, mainly due to the proliferation of localized skin-specific pigment cells, or caused by developmental abnormalities, which may be related to the living environment or nutritional deficiencies, and may be caused by other factors. Mostly in the 4th to 6th month of pregnancy, according to the differentiation process of the fetus, the birthmark will start to form at this time. However, there are no accurate statistics on the exact time of formation, and there are some fetuses that form birthmarks earlier or form birthmarks later, even before birth. Birthmarks generally do not affect the health of the body, so it is not recommended to treat them after birth.