Nystagmus in 3-month-old babies may be caused by congenital factors, hereditary factors or gene mutations. 1. Congenital factors: If the pregnant mother takes medication wrongly during pregnancy, which leads to adverse reaction of the fetus, or the delivery process is not smooth, which leads to the lack of oxygen of the newborn baby, it may cause nystagmus in 3-month-old babies. 2. Genetic factors: If the parents have nystagmus, it may be passed on to the baby, resulting in nystagmus in 3-month-old babies. 3. Genetic mutation: If the fetus has a genetic mutation, such as a lesion in the extraocular muscle system or visual system, the baby may also develop nystagmus after 3 months of life. Nystagmus in 3-month-old babies may cause visual impairment, such as vision loss, which will seriously affect the normal growth of babies and their future learning and life. Therefore, once your 3-month-old baby develops nystagmus, it is recommended to go to a regular hospital as soon as possible for consultation and examination, and take medical intervention in a timely manner.