Table 1 Participant inclusion and exclusion criteria

Congenital malformations

Congenital malformations are structural or chromosomal malformations that have a major impact on health and development.

Congenital gastrointestinal anomalies

Congenital gastrointestinal anomalies are mainly diagnosed by ultrasonography or radiography. Congenital gastrointestinal anomalies include gastric volvulus, cecal volvulus, and heterotopic pancreas.

Anorectal deformities

Anorectal deformities are a wide spectrum of abnormalities of the anus and rectum, which include anal atresia and rectovesical fistula. Anorectal deformities will be diagnosed by magnetic resonance or sonography.

Diarrhea

Diarrhea is defined as the occurrence of 3 or more loose stools within 24 h.

Intussusception

Intussusception is characterized by telescoping of one part of the gastrointestinal tract into another part, forming an obstruction. Intussusception will be diagnosed by ultrasound.

NEC

The main symptoms of NEC are abdominal distension and hematochezia, which are characterized by intestinal mucosal necrosis. NEC is mainly diagnosed by abdominal X-ray and ultrasound examination.

PDA

PDA will be confirmed by echocardiography.

Sepsis

Sepsis includes early onset and late onset. Early-onset sepsis is caused by pathogens transmitted from mothers, and late-onset sepsis is caused by nosocomial infections. Sepsis is defined as the overgrowth of bacteria in blood cultures in the presence of clinical deterioration.

Neutropenia

Absolute neutrophil count < 0.5 × 10⁹/L.

Coagulopathy

International standardization ratio > 1.4, partial thromboplastin time > 39 s, fibrinogen < 1.00 g/L, and platelet count < 100 × 10⁹/L.