A 3-D flexible bag to be filled with a biopharmaceutical product is formed by assembling of two wall elements and two gussets. At least one connection port is provided, for filling and/or emptying. A substantially parallelepipedal configuration is obtained in a filled state thanks to the unfolding of the gussets combined with the folding of flaps of the two wall elements. A transverse weld, formed at one end, connects the two wall elements of the bag and extends continuously, keeping in a folded-flat state: —；an elongate edge portion of one gusset； —；an elongate edge portion of the other gusset. This transverse weld has a length corresponding to a determined dimension of the flexible bag in the parallelepipedal configuration.
