4 weeks pregnant and took doxycycline for 3 days before realizing i was pregnant. should i worry?

Probably not: The antibiotic doxycycline is not known to cause pregnancy losses, congenital anomalies or birth defects when taken during the first trimester of pregnancy . 3 days of standard doses are not likely to cause harm. If doxycycline is used in high doses during the late second and third trimester , there have been reports of a theoretical risk of interference with the metabolism of bones or of teeth.