Which antibiotic should not be used during pregnancy due to the risk of causing discoloration of teeth in children?

Doxycycline is the antibiotic that should not be used during pregnancy primarily because of its potential to cause discoloration of teeth in developing children. During pregnancy, the fetal teeth are still developing, and the use of tetracycline antibiotics, like doxycycline, can lead to permanent staining or discoloration of the teeth. This is a result of the drug's ability to bind to calcium ions, which are essential in the formation of teeth.

The other antibiotics listed do not pose the same risk associated with dental discoloration during pregnancy. Ciprofloxacin and trimethoprim, while they have other contraindications in pregnancy, do not primarily affect tooth development like doxycycline does. Penicillin is generally considered safe and is routinely used in pregnant women when necessary since it does not have the same detrimental effects on children's teeth.