Final answer:
Valproate is the mood stabilizer with the worst teratogenic effects, including the risk of neural tube defects and cognitive impairments. It is considered unsafe for pregnant women or women who may become pregnant.
Step-by-step explanation:
The mood stabilizer known for having the worst teratogenic effect is Valproate (also known as Valproic acid or Divalproex sodium). This medication, while effective for mood stabilization in conditions such as bipolar disorder, poses significant risks during pregnancy. Among these risks are neural tube defects, including spina bifida, as well as other congenital malformations and cognitive impairments. For this reason, Valproate is generally not recommended for use in women of childbearing potential unless other treatments have been ineffective or are otherwise unacceptable.
It is extremely important for women who may become pregnant to discuss the potential risks and safer alternatives with their healthcare provider before starting on Valproate. The decision to take any medication during pregnancy should be informed by a careful consideration of both the benefits and the risks involved.