Before the placenta is formed and attaches to the uterine wall (8-12 weeks), you and the baby do not share a blood supply and so the medications that enter your bloodstream don't reach the baby. At 4 weeks, your baby is in no danger from neural tube defects, alcohol, or food borne pathogens.

Just to offer some peace of mind, I thought I was having a recurrence of ovarian cancer when I got pregnant the first time. I had a CT scan of my pelvis done at 4.5w and a radioactive IV contrast was used. I was also taking an opiod for control of chronic pelvic pain and a migraine medication known to cause birth defects, preterm labor, and miscarriage. I found out I was pregnant at 7.5w, stopped all meds and went on to have a healthy pregnancy and healthy full term baby.