There are two main types of headaches, tension headaches and migraine headaches.
Both are common in pregnancy, especially in the first trimester. Headaches are made worse by fatigue, dehydration, low blood sugar, and caffeine withdrawal.
Tylenol can be safely used in pregnancy to treat headaches. We should mention, in regards to Acetaminophen, that a recent study in 2014 saw a possible relation between long term Acetaminophen (Tylenol®) use and ADHD (attention-deficit hyperactivity disorder), especially after 20 weeks gestation. Short-term use for occasional colds and other ailments is still felt by most medical authorities to be safe while pregnant.
Most migraine relief medications as well as Ibuprofen (Advil®) (Motrin®) are not recommended in pregnancy. If your headache is very severe, you might be prescribed a stronger pain medication such as Codeine or Hydrocodone (Tylenol # 3®, Vicodin®).
The best way to avoid headaches is to keep a regular sleeping schedule, stay well hydrated, and eat regularly in order to maintain a steady glucose level in your body.
Severe headaches in the second half of your pregnancy, especially if persistent and associated with vision changes, could be signs of more severe problems. Contact you health care provider immediately if you notice this.