There are some medicines that are thought to be safe during pregnancy. . . but we keep learning more from research daily.
Prenatal exposure to acetaminophen (Tylenol) was associated with a significantly increased risk of attention-deficit/hyperactivity-like behavioral problems in children.
This was published in JAMA Pediatrics 2/24/14 and based on data from more than 60,000 children in Denmark. The risk was increased by 13% among those whose mothers had used acetaminophen overall during pregnancy. Although, when acetaminophen was used in the second and third trimesters, the risk increased by 44%. When acetaminophen used during all three trimesters, the risk of ADHD-like behaviors was increased by 24%.
It is postulated (guessed) that acetaminophen may alter maternal hormones which play critical roles in regulating fetal brain development.