Unconjugated hyperbilirubinemia in neonates: Etiology and pathogenesis

INTRODUCTION — Nearly all newborn infants develop elevated bilirubin levels (ie, total serum or plasma bilirubin [TSB] >1 mg/dL [17 micromol/L], which is the upper limit of normal for adults). As bilirubin levels increase, the newborn may develop visible jaundice. Newborns with severe hyperbilirubinemia (defined as TSB >25 mg/dL [428 micromol/L] in term and late preterm newborns [gestational age ≥35 weeks]) are at risk for developing bilirubin-induced neurotoxicity. Other related issues are discussed separately:

●Risk factors, clinical manifestations, and neurologic complications of neonatal hyperbilirubinemia (see "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications")

●Screening for hyperbilirubinemia in term and late preterm newborns (see "Unconjugated hyperbilirubinemia in term and late preterm newborns: Screening")

●Management of neonatal hyperbilirubinemia (see "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management" and "Unconjugated hyperbilirubinemia in term and late preterm newborns: Escalation of care")

●Hyperbilirubinemia in preterm infants (GA <35 weeks) (see "Unconjugated hyperbilirubinemia in preterm infants <35 weeks gestation")

●Conjugated (direct) hyperbilirubinemia in neonates (see "Causes of cholestasis in neonates and young infants")

DEFINITIONS

●Benign neonatal hyperbilirubinemia is a transient and normal increase in bilirubin levels occurring in nearly all newborn infants. It has also been referred to as "physiologic" jaundice.

●Severe neonatal hyperbilirubinemia is defined as a total serum or plasma bilirubin (TSB) >25 mg/dL (428 micromol/L). It is associated with an increased risk for developing bilirubin-induced neurotoxicity.

●Extreme neonatal hyperbilirubinemia is defined as a TSB >30 mg/dL (513 micromol/L). It is associated with a higher risk for developing bilirubin-induced neurologic dysfunction (BIND).

●Bilirubin-induced neurologic disorders (BIND) result from selective brain damage from free (unbound) bilirubin crossing the blood-brain barrier and binding to brain tissue. The spectrum of neurotoxic injury, including subtle dysfunction and acute and chronic bilirubin encephalopathy (ABE and CBE, respectively), is collectively referred to as BIND. The manifestations of BIND, ABE, and CBE are described separately. (See "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications", section on 'Consequences of severe hyperbilirubinemia'.)

BILIRUBIN METABOLISM — Knowledge of the basic steps in bilirubin metabolism is essential to the understanding of the pathogenesis of neonatal hyperbilirubinemia. Bilirubin metabolism is briefly reviewed here and is discussed in detail separately (figure 1). (See "Bilirubin metabolism".)

Bilirubin production — Bilirubin is a product of heme catabolism. In newborns, approximately 80 to 90 percent of bilirubin is produced during the breakdown of hemoglobin from red blood cells or from ineffective erythropoiesis. The remaining 10 to 20 percent is derived from the breakdown of other heme-containing proteins, such as cytochromes and catalase. Measurements of carbon monoxide (CO) production, such as end-tidal CO (ETCO) or carboxyhemoglobin (COHb) levels, both corrected for ambient CO (ETCOc and COHbc, respectively), can be used as indices of in vivo bilirubin production.

Bilirubin is produced in two steps (figure 1).

●The enzyme heme oxygenase (HO), located in all nucleated cells, catalyzes the breakdown of heme, resulting in the formation of equimolar quantities of iron, CO, and biliverdin.

●Biliverdin then is rapidly converted to bilirubin by the enzyme biliverdin reductase. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Screening", section on 'Determining follow-up and need for additional evaluation and/or treatment'.)

Bilirubin clearance and excretion — Clearance and excretion of bilirubin occur in the following subsequent steps (figure 1):

●Hepatic uptake – Circulating bilirubin bound to albumin is transported to the liver. Bilirubin then dissociates from albumin and is taken up by hepatocytes, where it is processed for excretion.

●Conjugation – In hepatocytes, the enzyme uridine diphosphogluconurate glucuronosyltransferase (UGT1A1) catalyzes the conjugation of bilirubin with glucuronic acid, producing bilirubin diglucuronides and, to a lesser degree, bilirubin monoglucuronides.

●Biliary excretion – Conjugated bilirubin, which is more water-soluble than unconjugated bilirubin, is secreted into the bile in an active process that depends upon canalicular transporters, and then excreted into the digestive tract (figure 2).

●Enterohepatic circulation – The secreted conjugated bilirubin cannot be reabsorbed by the intestinal epithelial cells. In the adult, it is reduced to urobilin by intestinal bacterial enzymes. However, at birth an infant's gut is sterile and, subsequently, infants have far fewer bacteria in the gut, so very little, if any, conjugated bilirubin is reduced to urobilin. In the infant, beta-glucuronidase in intestinal mucosa deconjugates the conjugated bilirubin. The unconjugated bilirubin can then be reabsorbed through the intestinal wall and recycled into the circulation, a process known as the enterohepatic circulation of bilirubin.

BENIGN NEONATAL HYPERBILIRUBINEMIA — Benign neonatal hyperbilirubinemia (also previously referred to as "physiologic jaundice") results in unconjugated (indirect-reacting) bilirubinemia that occurs in nearly all newborns [1]. It is a normal transitional phenomenon caused by the turnover of fetal red blood cells, the immaturity of the newborn's liver to efficiently metabolize (conjugate) bilirubin, and increased enterohepatic circulation.

Normal adult total serum or plasma bilirubin (TSB) levels are <1 mg/dL, while term newborns typically have TSB levels that peak at a median of approximately 8 to 9 mg/dL because:

●Newborns have more red blood cells (hematocrit between 50 to 60 percent), and fetal red blood cells have a shorter life span (approximately 85 days) than those in adults. After birth there is an increased turnover of fetal red blood cells, resulting in the production of more bilirubin.

●Bilirubin clearance (conjugation and excretion) is decreased in newborns, mainly due to the deficiency of the hepatic enzyme uridine diphosphogluconurate glucuronosyltransferase (UGT1A1). UGT1A1 activity in term infants at seven days of age is approximately 1 percent of that of the adult liver and does not reach adult levels until 14 weeks of age [2,3].

●There is an increase in the enterohepatic circulation of bilirubin as the amount of unconjugated bilirubin increases due to the limited bacterial conversion of conjugated bilirubin to urobilin, allowing for increased deconjugation by beta-glucuronidase in the intestinal mucosa (figure 1). This further increasing the bilirubin load in the infant. (See 'Bilirubin clearance and excretion' above.)

PEAK TSB LEVELS AND TIME TO RESOLUTION — Peak total serum or plasma bilirubin (TSB) and time to resolution vary by an infant's gestational age (GA), diet, and ethnicity/ancestry likely because of differences in hepatic uptake, clearance, and excretion [4-6]. Other important factors that impact the trajectory of neonatal hyperbilirubinemia include underlying hemolytic conditions (eg, alloimmune hemolytic disease, glucose-6-phosphate dehydrogenase deficiency) and individual vulnerability (eg, acute illness). (See "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications", section on 'Risk factors'.)

Newborns who do not follow the natural surge, plateau, and decline of TSB are most likely to require intervention. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

●Peak TSB – TSB levels peak at 48 to 96 hours of age, though in newborns of East Asian ancestry, the peak typically occurs between 72 and 120 hours of age. Mean peak TSB values typically range from 8 to 14 mg/dL (120 to 239 micromol/L). The 95^th percentile is approximately 18 mg/dL (308 micromol/L) [7].

●Time to resolution – Visible jaundice resolves within the first one to two weeks after birth. Clinical jaundice typically resolves by one week in formula-fed White and Black infants, and by the 10^th day in Eastern Asian infants. Jaundice resolves by three weeks in approximately 65 percent of exclusively breastfed newborns, although approximately one in five are still jaundiced at four weeks of age [8]. Persistence of hyperbilirubinemia beyond one to two weeks of age is generally considered prolonged hyperbilirubinemia/jaundice and these infants require an assessment of their direct or conjugated bilirubin levels to rule out cholestatic jaundice [9]. (See "Approach to evaluation of cholestasis in neonates and young infants".)

●Nomograms from different populations – Studies using transcutaneous bilirubin devices have developed nomograms to define the range of hour-specific normal values and the 75^th and 95^th percentiles according to postnatal age [10-14]. Many of the available studies were performed in populations that included predominantly breastfed White newborns. However, there are available data across different racial and ethnic groups and regions of the world [10-23]. All demonstrate an hourly progression to peak levels at ages 3 to 5 days and a transient plateau that is subsequently followed by decline. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Screening", section on 'Nomograms in different populations'.)

●Impact of genetic polymorphisms – Differences in peak TSB levels and time needed for resolution may result in part from genetic variability in hepatic conjugating ability of bilirubin [1]. As an example, polymorphisms in the UGT1A1 gene, due to differences in the number of thymine-adenine (TA) repeats or "TATA box" in the promoter region of the gene, vary among individuals of Eastern Asian, African, and European ancestries [24]. These polymorphisms correlate with decreases in UGT1A1 enzyme activity resulting in increased TSB levels and longer duration for resolution. (See "Gilbert syndrome", section on 'Genetics'.)

Another cause of variation results from a common genetic variant in the UGT1A1 gene at Gly71Arg (known as UGT1A1*6 polymorphism) that occurs commonly in individuals of Eastern Asian ancestry. This polymorphism is associated with increased risk of severe neonatal hyperbilirubinemia (by approximately 20 percent) [25,26]. Other UGT1A1 polymorphisms have also been identified (UGT1A1*9, UGT1A1*16, UGT1A1*27, and UGT1A1*28), but further work is needed to fully elucidate the impact of these mutations on an infant's risk for developing severe hyperbilirubinemia.

CAUSES OF SIGNIFICANT UNCONJUGATED NEONATAL HYPERBILIRUBINEMIA

Overview — Hyperbilirubinemia can be caused by specific underlying pathologic conditions or by exaggerations of the mechanisms responsible for normal neonatal "physiologic jaundice" jaundice (figure 1). Identification of an underlying pathologic cause of neonatal hyperbilirubinemia is useful in determining whether therapeutic interventions are needed and timing of intervention to prevent severe hyperbilirubinemia [27]. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

Any increase in bilirubin load resulting in significant hyperbilirubinemia is due to either or both an increase in bilirubin production or a decrease in bilirubin clearance (figure 3).

Increased production — The most common cause of clinically significant unconjugated hyperbilirubinemia is increased bilirubin production due to hemolytic disease processes, such as [24,25,28-33]:

●Isoimmune-mediated hemolysis (eg, ABO or Rh[D] incompatibility) [34]. (See "Alloimmune hemolytic disease of the newborn: Postnatal diagnosis and management".)

●Inherited red blood cell membrane defects (eg, hereditary spherocytosis and elliptocytosis). (See "Hereditary spherocytosis" and "Hereditary elliptocytosis and related disorders".)

●Erythrocyte enzymatic defects (eg, glucose-6-phosphate dehydrogenase [G6PD] deficiency [35], pyruvate kinase deficiency, and congenital erythropoietic porphyria). (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Pyruvate kinase deficiency" and "Congenital erythropoietic porphyria".)

●Sepsis – The mechanism is not known; however, one theory suggests that increased oxidative stress due to sepsis damages neonatal red blood cells [28]. (See "Clinical features, evaluation, and diagnosis of sepsis in term and late preterm neonates".)

●Other causes of increased bilirubin production due to increased red blood cell breakdown include polycythemia or sequestration of blood within a closed space, such as in cephalohematomas. (See "Neonatal polycythemia".)

●Macrosomic infants of diabetic mothers (IDM) have increased bilirubin production due to either polycythemia or ineffective erythropoiesis. (See "Infants of mothers with diabetes (IMD)".)

Decreased clearance — Inherited defects in the gene that encodes the enzyme UGT1A1, which catalyzes the conjugation of bilirubin with glucuronic acid, decrease bilirubin conjugation. This reduces hepatic bilirubin clearance and increases total serum or plasma bilirubin (TSB) levels [36]. These disorders include Crigler-Najjar syndrome types I and II and Gilbert syndrome. These syndromes are briefly summarized below and discussed in detail separately. (See "Crigler-Najjar syndrome" and "Gilbert syndrome".)

Crigler-Najjar syndrome — There are two variants of Crigler-Najjar syndrome. (See "Crigler-Najjar syndrome".)

●Crigler-Najjar syndrome type I (CN-I) – This is the most severe form of inherited UGT1A1 disorders. UGT1A1 activity is essentially absent, and severe hyperbilirubinemia develops in the first two to three days after birth. Lifelong phototherapy is required to avoid the development of bilirubin-induced neurologic dysfunction (BIND) unless liver transplantation is performed. The mode of inheritance is autosomal recessive.

●Crigler-Najjar syndrome type II (CN-II) – CN-II is less severe than is CN-I. UGT1A1 activity is low but detectable. Although some affected children develop severe jaundice, the hyperbilirubinemia often responds to phenobarbital treatment. CN-II usually is inherited in an autosomal recessive manner, although autosomal dominant transmission occurs in some cases.

Gilbert syndrome — Gilbert syndrome is the most common inherited disorder of bilirubin glucuronidation due to mutations in the UGT1A1 gene. In White and Black patients, it results from a mutation in the promoter region of the UGT1A1 gene [37]. The mutation causes a reduced production of UGT1A1, leading to unconjugated hyperbilirubinemia. In the Eastern Asian population, Gilbert syndrome results from a missense mutation in the coding area of the UGT1A1 gene [25,38]. Breast milk jaundice during the second week after birth may be due to the concurrent neonatal manifestation of Gilbert syndrome. (See 'Breast milk jaundice' below.)

In the United States, 9 percent of the population is homozygous and 42 percent heterozygous for the Gilbert mutation [39]. Newborns who are homozygous for the gene mutation have a higher incidence of developing hyperbilirubinemia during the first two days after birth than infants without the mutation or those who are heterozygous [40]. Similar findings have been noted in other parts of the world, especially in Eastern Asian countries [26,41]. There is also evidence that increased hemolysis contributes to neonatal hyperbilirubinemia in addition to the reduction in bilirubin conjugation [42].

However, data from case series have reported that the Gilbert genotype alone is not sufficient to increase the incidence of hyperbilirubinemia [43,44]. Rather, the Gilbert genotype appears to only become clinically relevant when affected newborns have increased bilirubin production or enhanced enterohepatic circulation of bilirubin [45,46]. In particular, the combination of a Gilbert genotype with an underlying condition that increases TSB production, such as G6PD deficiency, is associated with severe or significant hyperbilirubinemia [33,45,47]. (See "Gilbert syndrome".)

OATP-2 polymorphism — In addition to the polymorphisms of the UGT1A1 discussed above, a study of Taiwanese newborns reported that those with a polymorphic variant of the organic anion transporter protein OATP-2 (also known as OATP-C or solute carrier organic anion transporter 1B1 [SLCO1B1]) were more likely to develop severe hyperbilirubinemia [48]. Furthermore, the combination of the OATP-2 polymorphism with a UGT1A1 gene mutation increased this risk.

Other causes of decreased clearance — Other causes of decreased bilirubin clearance include maternal diabetes [49], congenital hypothyroidism, galactosemia, and panhypopituitarism, although in the latter two conditions, infants typically present with elevated conjugated hyperbilirubinemia. These disorders usually are identified by metabolic screening programs; however, infants may develop severe and prolonged jaundice before screening results become available. (See "Clinical features and detection of congenital hypothyroidism" and "Galactosemia: Clinical features and diagnosis".)

Increased enterohepatic circulation of bilirubin — Causes of hyperbilirubinemia due to increased enterohepatic circulation of bilirubin include impaired intestinal motility caused by functional or anatomic obstruction and possibly breast milk jaundice, but the underlying mechanisms of the latter has not been confirmed.

Breast milk jaundice — Breast milk jaundice is defined as the persistence of benign neonatal hyperbilirubinemia beyond the first two to three weeks of age. It typically presents after the first three to five days of life, peaks within two weeks after birth, and progressively declines to normal levels over 3 to 12 weeks [8,50]. Breast milk jaundice needs to be distinguished from breastfeeding (lactation) failure jaundice, the latter is due to suboptimal fluid and caloric intake during the first seven days of life. (See 'Inadequate milk intake' below.)

In infants with jaundice who exclusively receive human milk, TSB levels >5 mg/dL (86 micromol/L) often persist for several weeks after delivery [8]. Although the hyperbilirubinemia is generally mild and typically does not require intervention, it should be monitored to ensure that it remains in the unconjugated form and does not increase. If TSB levels begin to increase or there is a significant component of conjugated bilirubin, evaluation for other causes of hyperbilirubinemia should be performed. In the case of elevated conjugated bilirubin, causes of cholestasis need to be considered. If after evaluation human milk intake is the only remaining viable factor, human milk feeding can continue if the TSB remains in a safe zone with the expectation of resolution by 12 weeks of age [51]. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Screening" and "Causes of cholestasis in neonates and young infants" and "Approach to evaluation of cholestasis in neonates and young infants".)

The underlying mechanism of "breast milk jaundice" is not fully known. Human milk contains high concentrations of beta-glucuronidase, which catalyzes the hydrolysis of beta-D-glucuronic acid [52]. In contrast, there is negligible amounts of beta-glucuronidase in infant formula, and formula-fed infants have lower levels of bilirubin than those who receive human milk [53,54]. For breast milk-fed infants, the loss of beta-D-glucuronic acid due to increased degradation is thought to promote an increase in intestinal absorption of unconjugated bilirubin [52] (figure 1). Beta-glucuronidase inhibitors, such as enzymatically-hydrolyzed casein or L-aspartic acid, which is contained in casein hydrolysate formula, have been used prophylactically in breastfed newborns [55]. However, the prolonged unconjugated hyperbilirubinemia associated with human milk is benign, and there appears to be no benefit for the use of these agents [56]. As a result, we do not recommend these agents for treating breast milk jaundice.

Another potential underlying mechanism is polymorphic mutation of the UGT1A1 gene. In a Japanese study of 170 neonates with breast milk jaundice, half of infants were homozygous for the UGT1A1*6 genotype [57]. These infants had higher TSB levels than infants with other polymorphisms. The UGT1A1*6 genotype was not detected in control infants. However, further studies in other areas of the world are needed to determine whether there is a causal relationship between genetic variation of the UGT1A1 gene and breast milk jaundice. Thus, currently genetic testing should not be used in the evaluation of breast milk-related jaundice.

Ileus or intestinal obstruction — Ileus or anatomic causes of intestinal obstruction increase the enterohepatic circulation of bilirubin and result in hyperbilirubinemia. TSB levels are frequently higher with small bowel than with large bowel obstructions. As an example, hyperbilirubinemia occurs in 10 to 25 percent of infants with pyloric stenosis when vomiting begins. (See "Infantile hypertrophic pyloric stenosis", section on 'Clinical associations'.)

Inadequate milk intake — Breastfeeding difficulties are common during the first week after birth. These difficulties can lead to inadequate intake of fluids and calories resulting in hypovolemia and significant weight loss. This results in hyperbilirubinemia (jaundice) and, in some cases, hypernatremia (serum sodium >150 mEq/L). Inadequate intake also causes slower bilirubin elimination and increases enterohepatic circulation of bilirubin that contribute to an elevated TSB level. (See "Common problems of breastfeeding and weaning", section on 'Inadequate milk intake' and "Initiation of breastfeeding", section on 'Assessment of intake'.)

Maternal breastfeeding complications (eg, engorgement, cracked nipples, and fatigue) compounded by neonatal factors (eg, ineffective suck), can contribute to ineffective breastfeeding if they are not properly addressed prior to hospital discharge. (See "Common problems of breastfeeding and weaning".)

Late preterm infants (defined as gestational age [GA] between 34 and 36 weeks and 6 days) are more likely to experience difficulty in establishing successful breastfeeding than term infants. Late preterm infants may not fully empty the breast because of increased sleepiness, fatigue, and/or difficulty maintaining a latch because their oro-buccal coordination and swallowing mechanisms are not fully matured. As a result, additional support and close monitoring are warranted for this group of infants and their mothers. (See "Breastfeeding the preterm infant", section on 'Late preterm infants'.)

Establishment of successful breastfeeding, one of the mainstays of preventing hyperbilirubinemia, is challenging due to shortened postpartum length of stay for newborn infants and their mothers. Postnatal education, support, and care should be provided to the infant-mother dyad during the birth hospitalization and after discharge and is discussed in greater detail separately. (See "Breastfeeding: Parental education and support" and "Initiation of breastfeeding".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Neonatal jaundice".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Jaundice in babies (The Basics)")

●Beyond the Basics topic (see "Patient education: Jaundice in newborn infants (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Importance – Nearly all term and near-term newborn infants develop elevated bilirubin levels after birth. Infants with severe hyperbilirubinemia (defined as total serum or plasma bilirubin [TSB] >25 mg/dL [428 micromol/L]) are at risk for developing bilirubin-induced neurologic dysfunction (BIND). (See 'Definitions' above and "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications", section on 'Consequences of severe hyperbilirubinemia'.)

●Benign hyperbilirubinemia – Benign neonatal hyperbilirubinemia, previously referred to as "physiologic jaundice," is a normal transitional phenomenon caused by the turnover of fetal red blood cells, immaturity of the newborn's liver to efficiently metabolize bilirubin, and sterile newborn gut, resulting in decreased bilirubin clearance and increased enterohepatic circulation (figure 1). It typically presents as mild unconjugated hyperbilirubinemia. TSB levels usually peak at 48 to 96 hours of age, though in newborns of East Asian ancestry, the peak typically occurs between 72 and 120 hours of age. Mean peak TSB values typically range from 8 to 14 mg/dL (120 to 239 micromol/L). Visible jaundice resolves within the first one to two weeks after birth. (See 'Benign neonatal hyperbilirubinemia' above and 'Peak TSB levels and time to resolution' above.)

●Mechanism of neonatal hyperbilirubinemia – Hyperbilirubinemia is due to increased bilirubin load either due to an increase in bilirubin production or decrease in clearance, or both (figure 3). (See 'Increased production' above and 'Decreased clearance' above.)

●Causes of clinically significant hyperbilirubinemia – For infants with clinically significant hyperbilirubinemia, identification of the underlying cause is useful in determining the need for and timing of therapeutic interventions. (See 'Causes of significant unconjugated neonatal hyperbilirubinemia' above.)

Causes of significant unconjugated neonatal hyperbilirubinemia can be classified by pathogenesis as follows:

•Increased production of bilirubin (see 'Increased production' above):

-Hemolytic disease (see "Alloimmune hemolytic disease of the newborn: Postnatal diagnosis and management" and "Overview of hemolytic anemias in children")

-Polycythemia (see "Neonatal polycythemia")

-Cephalohematoma (see "Neonatal birth injuries", section on 'Cephalohematoma')

-Sepsis (see "Clinical features, evaluation, and diagnosis of sepsis in term and late preterm neonates")

•Increased enterohepatic circulation of bilirubin (see 'Increased enterohepatic circulation of bilirubin' above):

-Breast milk jaundice (see 'Breast milk jaundice' above)

-Impaired intestinal motility (ileus or obstruction)

•Inadequate intake of breast milk due to breastfeeding difficulties resulting in hypovolemia and inadequate caloric intake. (See "Common problems of breastfeeding and weaning", section on 'Inadequate milk intake'.)