 David Stevenson, onye duru ndi otu ya na ụlọọrụ nchọpụta a kpọrọ, March of Dimes Prematurity Research Centre nke dị na mahadum Stanford dị na mba Amerịka kọwara usoro ọhụrụ ahụ bụ ụzọ iji gee nti na mkparitaụka na-aga netiti nne, nwa nakwa alọnwa nke ụfọdụ ma dịka ichi placenta na enyeghị afọime nsogbu ọbụla. 