Maria, a 28-year-old woman, is in her first trimester of pregnancy. She has not been taking prenatal vitamins, and her diet lacks sufficient folic acid. During her routine checkup, her doctor informs her that a deficiency in folic acid during pregnancy can lead to serious birth defects in the baby. One of the most common defects associated with folic acid deficiency is a condition where the brain or spinal cord fails to develop properly.

Based on the case study, which birth defect is most commonly associated with maternal folic acid deficiency during pregnancy?