"To estimate the risk of miscarriage among asymptomatic women after a prenatal visit between 6 and 11 weeks of gestation where proof of fetal viability of a singleton was obtained by office ultrasonography at the same visit. METHODS: Those recruited were 697 asymptomatic women who attended their first antenatal visit between 6 (+2 days) and 11(+6 days) weeks of gestation, where evidence of fetal cardiac activity of a singleton was obtained by office ultrasonography. RESULTS: The risk of miscarriage among the entire cohort was 11 of 696 (1.6%). The risk fell rapidly with advancing gestation; 9.4% at 6 (completed) weeks of gestation, 4.2% at 7 weeks, 1.5% at 8 weeks, 0.5% at 9 weeks and 0.7% at 10 weeks."

http://www.ncbi.nlm.nih.gov/pubmed/18310375

Much more info here: https://sites.google.com/site/miscarriageresearch/miscarriage-general