Creating an Advanced Search
The Advanced Search page can help you build a search with multiple concepts.
On the Advanced Search screen, you can see your previous searches under "History and Search Details".
In this example, one of the terms is too narrow, while the other one is too broad.
To make your search results more specific, you can combine search sets by clicking the set number under "Search." When you click it, you'll see several choices:
Use "OR in builder" to combine search sets which are synonyms - for instance, pregnancy OR prenatal
Use "AND in builder" to add each individual concept to the search - (pregnancy OR prenatal) AND (hypertension or antihypertensive agents)
Use "NOT in builder" to eliminate an idea from your search - (pregnancy OR prenatal) AND (hypertension or antihypertensive agents) NOT animals
Using Brackets
Note that bracket placement matters.
(A AND B) OR (C AND (D OR E))
is different from
(A AND B OR (C AND (D OR E)))
PubMed will read any search from left to right unless otherwise directed by brackets per Order of Operations Rules.
Example: A search for ("early pregnancy" OR "prenatal") AND (surgery OR operation) will return articles on all these topics:
early pregnancy AND surgery
prenatal AND surgery
prenatal AND operation
early pregnancy AND operation
But the same search without the brackets will return results for articles on:
early pregnancy AND surgery
prenatal AND surgery
operation
The second example finds many more results, because PubMed is finding any article that uses the word operation.