FWIW I had a pretty much constant headache for the best part of my 2nd tri (from about 12 to 22ish weeks) and nothing helped, not even panadol. Dr Google told me that it's not that uncommon (but more common in the 1st tri) and is due to hormonal imbalances. My OB told me it is because I have low/ish blood pressure (110/60). He suggested that I lay down and rest when I had one (like that was going to happen with a toddler to look after) and that if I couldn't, to eat something salty to get my blood pressure back up. Amazingly the salty food did help.