A new study published in Archives de Pediatrie evaluated the dose-dependent effect of human milk fortifier (HMF) on the osmolality of various preterm human milks (PHMs) at different gestational and lactation stages, and with different storage and treatment conditions that are routinely used in neonatal intensive care units (NICUs). This was a prospective, multicenter, comparative, and non-interventional study, wherein 24 mothers who had given birth before 28 weeks or between 29 and 31 weeks of gestation, participated after 1-2 weeks or 3-4 weeks of breastfeeding post-delivery. Osmolality of fresh or pasteurized human milk stored at 4°C was measured (cryoscopy) at baseline, and 24 hours after adding a human milk fortifier at 3%, 4%, 4.5%, and 5% (w/v). It was found that PHM without supplementation had an osmolality of 301±8 mOsm/kgH2O. Adding 3-5% human milk fortifier induced a linear increase of osmolality. Whereas, with 4% human milk fortifier, the osmolality measure was 443±13mOsm/kg H2O. Neither a 24-hour storage at 4°C nor pasteurization induced a modification of osmolality compared to the fresh samples. Hence, it was inferred that irrespective of the origin and quality of milk and hospital practices, adding up to 4% (w/v) of human milk fortifier to PHM increases its nutritional quality and osmolality within the safety limits.