At present, the causes of harelip are not yet completely clear, and may be related to genetic factors, maternal factors, toxic factors and so on. 1. Genetic factors: If the parents have hare lip condition, then the chance of the children having hare lip will be greatly increased, this is because there is a gene mutation associated with hare lip. 2. Maternal factors: During pregnancy, malnutrition, lack of vitamins, viral infections, bacterial infections, etc., may cause the baby’s oxygen supply is insufficient, resulting in the occurrence of harelip. 3. Poisoning factors: During pregnancy, if you take certain medications, such as Vitamin A acid, because these substances have toxic side effects on the fetus, it may lead to underdevelopment of the fetus and the appearance of harelip. After pregnancy, pregnant women need to go to the hospital regularly for pregnancy checkups to understand the development of the fetus in order to reduce the chances of harelip.