Black lines on the breasts during pregnancy are mainly considered to be caused by physiological estrogen and progesterone changes, in addition to infections, skin diseases, etc. It is recommended to consult a doctor for timely diagnosis. 1. Physiological factors: the increase in estrogen and progesterone during pregnancy can lead to breast enlargement and pigmentation, causing black lines on the breasts. This is generally a normal phenomenon, and will gradually decrease or subside after delivery. 2. Pathological factors: eczema, fungal infections, allergies, etc. can also lead to the appearance of black lines on the breasts, usually accompanied by itching symptoms, it is recommended that you do not scratch, and timely medical treatment. To summarize, the appearance of black lines on the breasts during pregnancy is mostly considered to be caused by estrogen and progesterone, but in a few cases, it can be caused by infection, allergy and other reasons. In a few cases, it can be due to infection, allergy and other reasons. It is recommended to seek medical advice and scientific treatment.