Health Sciences - Basic Library Research

The most obvious search terms are the words you are already using to describe the topic.

'pregnancy in middle aged women' = 'pregnancy', 'middle aged', and 'women'

'health insurance coverage of lower income residents of cities' = 'health insurance', 'lower income', and 'cities'

Think of alternative terms or synonyms for the words you have (and don't worry about matching nouns to nouns; use whatever term comes to mind):

'cities' = 'urban'

'teenagers' = 'adolescents'

Also think of broader and narrower terms:

'lower income' = 'poverty' or 'poor', a broader term might be 'socioeconomic', i.e. a study that compares people of different 'socioeconomic' statuses would probably include poor people.