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.
Try this automated search strategy builder to format you search terms into a useful search that can be pasted into most databases.
Read some background information on the topic and get words related to the subject from the readings.