It is absolutely safe to start taking folic acid during the second trimester of pregnancy. In fact, it is recommended that women take folic acid supplements throughout their entire pregnancy. Folic acid plays a crucial role in the development of your baby's neural tube, which eventually becomes their brain and spinal cord. By starting to take folic acid at 15 weeks, you are ensuring that your baby receives this essential nutrient for proper growth and development. There are no known negative effects associated with taking folic acid during pregnancy, so you can rest assured that you are doing something beneficial for your baby's health. Remember to follow the recommended dosage provided by your healthcare provider and maintain a healthy diet rich in folate as well.