Most efficacious within 72 hours of unprotected sex.
And according to this review article (in a neuro journal of all places!), "there is no evidence that hormonal emergency contraception damages the fetus either in an established pregnancy or in the case of treatment failure."
Nonetheless, given that all hormonal emergency contraceptives are pregnancy safety category X, I believe they should be used with extreme caution if there is any concern for established pregnancy.
Source
Frye, C. "An overview of oral contraceptives" Neurology 2006.