Doxycyclene is safe for the queen, but not her developing fetuses or nursing babies. They do not have viable immune systems yet and any antibiotic could result in abortion, still births, or deformities. The purpose of any antibiotics is to kill bacteria but it is non discriminatory. That is it kills both the bacteria cause the queen's URI as well as beneficial bacteria needed for the kittens to grown and develop. Just as with pregnant humans, it is always a judgement call as to whether the benefits warrants the risk.
Also it can cause the kittens to have weird looking teeth, fluorescent yellow. Get a medication from the vet.
Hi jjackson, you can give some people meds like tums to cats but antibiotics need to be prescribed from the vet only because the dosage and type of infections cats get are different from the types we get and it can be lethal to give people meds to cats especially a pregnant one. The antibiotic you mention is in the tetracycline family and can cause birth defects in humans so do not give it to your cat, take her to the vet.