A 28-year-old woman with bipolar I disorder, currently stable on valproate 1000 mg daily, presents for a preconception counseling visit. She and her partner plan to conceive within the next six months. Her psychiatric history includes three manic episodes, two of which required hospitalization, and she has been stable on valproate for two years. She asks the PMHNP about the reproductive safety profile of her current medication. Which of the following statements most accurately reflects current evidence regarding valproate exposure during pregnancy?