Recently, a new formulation of follitropin alfa has been developed as the next step in innovation in Serono capitalized on the fact that Serono recognized that follitropin alfa manufacturing was highly controlled and with a controlled specific activity that allowed filling the product by mass guaranteeing the dose being delivered. Until now follitropin alfa has been produced as a lyophilisate for injection (either as single dose or multidose applications) in glass ampoules or in glass vials and administered using syringes. Today, Gonal-f® fill-by-mass is available as a new liquid formulation that can be administered with the pen device which is prefilled and hence the subject does not have to assemble the device making it simpler to use.