Hi thanks for your question.
Some medicine are known for their teratogenic effect ( damage caused to fetus by drug) and tetracycline
group of drugs to which doxy
cyline belongs causes teeth discolouration and permenent teeth damage if taken during pregnancy
.There is no controlled study to show teratogenic effects of doxycycline in humans,however it has produced teratogenic effects(musculo skeltal defect) in study done on mice.In Hungarian case control surveillance of congential abnormalties Doxycycline was taken by only 0.3% women during their pregnancies who had given birth to congenitally abnormal newborn.
The botomline is there is no proved teratogenic effects of doxycycline .However it should be taken during pregnancy only when no other substitute is available and benifits outnumbers the risks.
Hope this answers your question.