In a study published in the August 2008 issue of BJOG: An International Journal of Obstetrics & Gynaecology, the conclusion reached by the researchers is that rubbing cocoa butter lotion over the abdomen, breasts and thighs during pregnancy does not prevent striae gravidarum aka stretch marks.

Below is a snippet from the abstract,

"Of 210 women enrolled, 175 (83%) completed the study. Ninety-one women received the study lotion and 84 received the placebo. There was no difference in the development of SG (45.1% versus 48.8%; P = 0.730) or the severity of SG between cases and controls. The results did not change when presence of stretch marks at enrolment or compliance with the regimen were taken into account."