Medicines having molecular weights below 500 g/mol may easily pass the placenta and reach the developing baby. Medicines having molecular weights over 500 g/mol have a more difficult time crossing the placenta, while medications with molecular weights above 1000 g/mol are unable to pass the placental barrier at all. Many commonly used antibiotics are small molecules (those with a molecular weight of less than 500 g/mol) and they are capable of passing through the placenta.