Birth weight loss is a very important issue but is little attended between pediatricians and neonatologists. It affects healthy term neonates, breastfed exclusively and without risk factors, in their first 10 days old. It is accepted that it is due to low milk intake secondary to a "bad breastfeeding technique" but little is accepted that the true origin of this weight loss is the dehydration by low milk intake and that this is secondary to Delayed Lactogenesis II (DLII), rather than to a "Bad Breastfeeding Technique" (BBT). The combination of mothers with low milk production and dehydrated infants constitutes the little-known "Insufficient Milk Syndrome" (IMS), [1].

Has been considered normal birth weight loss up to 10%, in the last 20 years, attributed to "redistribution of extracellular fluid", but without scientific evidence sufficient for this. The World Health Organization (WHO), the American Academy of Pediatrics (AAP), and the Breastfeeding Medicine Academy (BMA) accept as normal a loss of 7% to 8% of their birth weight [2], which brought to the design of charts [3] and reference nomograms of the "degree of normality" of this weight loss, which are used to detect newborns in risk hypernatremic dehydration [4].

We believe that neonates should not have birth weight loss (BWL), based on the universal knowledge that Neonatal Growth and Development (NGD) are progressive. Therefore, if we accept that BWL is normal, we also accept that NGD is regressive. Historic and universally we know that healthy neonates should gain 15 to 30 g of weight per day from birth, if they receive, at least, a Liquid of 70 ml/kg/day and a Caloric 70 KCal/kg/day supply from the first day of life, which is called "neonatal Hydro-Caloric Requirement" (nHCR) with hydric/caloric ratio of 1:1 [5].

The nHCR increases 20 ml and 20 KCal per kg per day in the first 5 days, the milk supply should increase at the same rate to achieve an average weight gain of 20 g/day. Therefore, a 3 kg neonate at birth should weigh 200 g more at 10 days old (ranks 150 to 300 g). That is, they must increase, on average 6.6% of their birth weight (ranks 5 to 10%) to consider that NGD is progressive.

We must remember that Lactogenesis II starts 24 to 48 hrs after the placenta is expelled. In the first 7 days, the first milk production is "colostrum", from 20 ml to 100 ml/day, with a Sodium concentration (Na) of 22 mmol/L. The second production is "transition milk", from 500 to 750 ml/day, between the 8 and 14 days, with Na of 14 mmol/L. Finally, the third production is "mature milk", from 1200 to 1800 ml/day, with Na of 7 mmol/L from 15 days of breastfeeding onward. Therefore, colostrum and transitional milk are hypernatremic in relation to mature milk, which to the newborn causes thirst and dehydration due to an osmotic effect to preserve its circulatory volume, which is reduced precisely by low milk intake, minor to its nHCR in its first 5 to 10 days of life manifested with BWL [6].

We do not doubt that colostrum provides "quality" of nutritional, immunological, and affective type, but, in the first 10 to 15 days of breastfeeding, the "quantity" or volume, is insufficient to satisfy the nHCR because, colostrum production does not increase with the same speed that increases the nHCR, with which keeping the newborn with the low hydric supply by low intake and, therefore, "surviving dehydrated" [1,5,6].

With the maximum expected volume of colostrum produced (VCP) of 100 ml/day, the hydric supply that a mother provides to a baby of 3 kg of birth weight is only 33 ml/kg/day, which represents less than half of the nHCR that is, in the first day of life, 70-80 ml/kg/day. Our average VCP is 40.4-44.6 ml/day [7] between 750 primiparous and multiparous women [1,5-7). With the expected VCP, hydric balance is negative in 37 ml/kg/d in the first 24 hours. The nHCR increases 20 ml/kg/day the first 5 days of life. That is 70, 90, 110, 130, 150/kg/d corresponding, in ml/day (volume), to 210, 270, 330, 390, 450 ml. If the colostrum production increases 100 ml/day, that correspond in ml/day (volume), to 100, 200, 300, 400, 500 ml, the hydric balance day by day is negative, which we called "Cumulated Hydric Deficit" (CHD). By day 5 the total is 150 ml that corresponds to 5% of birth weight.

With the maximum nHCR, the volume is 80, 100, 120, 140, 160 ml/kg/d, which represents, in ml/day, 240, 300, 360, 420, 480 ml. With this volume, the hydric balance is negative in 140, 100, 60, 20 ml in the first 4 days. By day 5 CHD is negative of 300 ml, which represents and explains the 10% BWL, or major in the following days of life. This supports our theory which says, "dehydration is the real cause of the BWL, due to low colostrum intake, which in turn, is secondary to DLII, both members of the IMS [5-7].

We want to address and highlight this issue, because hospital readmission of neonates in their first 10 days of life, with jaundice, fever, and weight loss, is progressively increasing, in whom is not recognize dehydration, because "they seem healthy". Until the dehydration, is very evident, progressive, and hypernatremic, neonates arrive at the emergency room whit moderate and serious dehydration, and they are diagnosed as "septic" because the clinical picture is similar to "sepsis" [1,5,6].

Neonatal weight loss due to dehydration is totally preventable if it is detected on time. However, the "current detection methods" do not recognize it until the neonates are severely dehydrated, with a weight loss of 10% or more, which delays the intervention that currently mainly focused on "correcting the breastfeeding technique" which is not effective because the cause is not this, the real cause is the DLII, therefore, formula feeding should be allowed while "the milk coming in" in mothers, which happens "physiologically", at the end of the second week of exclusive breastfeeding.

To detect early, infants with any degree of dehydration, exist the "Neonatal Dehydration Criteria" (NDC), allows us to find dehydrated neonates by low milk intake that, usually are not detected because the signs of neonatal dehydration are not recognized because the babies "seem healthy" or “look good” even the birth weight loss is considered normal. The NDC is a practical tool, no-cost, novel, and easy to apply prior to the discharge of the newborn, and in follow-up consultations during the neonatal period, even beyond this period [7] (Table 1).

Table 1: Neonatoa Dehydration Criteria Registration sheet.