Who responds to postal questionnaires?
D001719:Birth Certificates; D005006:Ethnicity; D005260:Female; D006753:Hospital Records; D006801:Humans; D007231:Infant, Newborn; D009774:Obstetrics; D011177:Postal Service; D011247:Pregnancy; D012923:Social Class; D011795:Surveys and Questionnaires; D006113:United Kingdom