title: The fertility riddle.
authors.forename: Declan
authors.initials: D
authors.lastname: Butler
mesh_terms: D001723:Birth Rate / Q000208:ethnology / Q000639:trends*; D005260:Female; D005298:Fertility / Q000502:physiology*; D005500:Follow-Up Studies; D006801:Humans; D007246:Infertility / Q000453:epidemiology* / Q000209:etiology*; D008297:Male; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D018456:Reproductive Medicine / Q000379:methods / Q000639:trends; D012189:Retrospective Studies; D013076:Sperm Count / Q000706:statistics & numerical data; D018433:Twin Studies as Topic