Amphetamines are stimulatory drugs that are highly addictive. It is highly recommended to not use Methamphetamines while pregnant.
It is unlikely that the pure form causes any malformations in your baby but chemical additives are commonly used by drug dealers to expand drug volume, and these additives may cause birth defects.
Amphetamines seem to, however, affect the growth of the baby and may be associated with premature delivery. Finally, if commonly used, it appears to be associated with neonatal and childhood muscle and psychological issues.