Objective:To investigate the incidence, influencing factors, and clinical characteristics during hospitalization of small for gestational age (SGA) infants among very preterm infants (VPIs) in neonatal intensive care units (NICUs).Methods:This study was a multicenter cohort study. Clinical data of VPIs with gestational age<32 weeks admitted to 19 collaborative units from January 1, 2019 to December 31, 2022, were collected from the Neonatal Perinatal Collaborative Network of Suxinyun (SNPN) and analyzed. General characteristics, perinatal conditions, and clinical manifestations during hospitalization of SGA infants among the VPIs were analyzed using Chi-square test, Mann-Whitney U test, and multivariate logistic regression. Clinical characteristics of symmetric and asymmetric SGA infants with hospital stay >7 d were also analyzed. Results:(1) During the study period, a total of 5 045 VPIs were included, among which there were 346 large for gestational age (LGA) infants, 4 475 appropriate for gestational age (AGA) infants, and 224 SGA infants, with a SGA incidence of 4.4%. The incidence of SGA was significantly higher in VPIs born at 30-31 +6 gestational weeks than in those with gestational age<28 weeks and born at 28-29 +6 weeks [6.1% (145/2 380) vs. 3.0% (25/833) and 3.6% (54/1 486), χ 2=11.77 and 11.32, both P<0.001]. No significant difference in SGA incidence was found between VPIs with gestational age<28 weeks and those born at 28-29 +6 weeks ( χ 2=0.65, P=0.248). (2) Multivariate logistic regression analysis showed that multiple pregnancies, hypertensive disorders of pregnancy (HDP) and assisted reproductive technology (ART) were independent risk factors for the occurrence of SGA in VPIs [ OR values (95% CI): 1.94 (1.43-2.64), 7.06 (5.34-9.33), and 1.59 (1.14-2.23)], and there was a significant gender difference, with the incidence of SGA being significantly lower in males than in females ( OR=0.61, 95% CI: 0.46-0.81). (3) The body temperature of SGA infants on admission was lower than that of AGA infants [36.0 °C (35.5-36.5 °C) vs. 36.0 °C (35.8-36.5 °C), Z=-2.08, P=0.004]. Moreover, the fasting time, the duration of parenteral nutrition, and the length of hospital stay for SGA infants were longer [3 d (1-7 d) vs. 2 d (1-5 d), 24 d (16-34 d) vs. 19 d (13-29 d), and 47 d (37-61 d) vs. 42 d (30-58 d), Z=-4.13,-4.65, and -3.02, all P<0.05]. The incidence of feeding intolerance, hypoglycemia, neonatal parenteral nutrition-associated cholestasis, and extrauterine growth restriction (EUGR), and the rate of treatment withdrawal or death were also higher in SGA infants [69.8% (143/205) vs. 58.9% (2 450/4 157), 17.6% (36/205) vs. 7.0% (292/4 157), 13.7% (28/205) vs. 6.4% (265/4 157), 77.7% (159/205) vs. 55.2% (2 295/4 157), and 7.8% (16/205) vs. 3.9% (162/4 157), χ 2=9.49, 31.19, 16.54, 44.40, and 7.62, all P<0.05]. Among SGA infants with hospital stay >7 d, there were 111 cases (54.1%) of symmetric SGA and 94 cases (45.9%) of asymmetric SGA. Compared with symmetric SGA infants, asymmetric SGA infants had shorter body length [34 cm (32-36 cm) vs. 38 cm (36-40 cm), Z=-8.49] and lower Apgar score at 1 min [7 points (5-8 points) vs. 8 points (5-8 points), Z=-3.05]. Besides, the proportion of multiple pregnancies and the incidence of postnatal hypoglycemia were higher in asymmetric SGA cases [38.3% (36/94) vs. 21.6% (24/111), 24.5% (23/94) vs. 10.8% (12/111), χ 2=6.84 and 6.71, both P<0.05], while the incidence of feeding intolerance and EUGR during hospitalization was lower [61.7% (58/94) vs. 76.6% (85/111), 58.5% (55/94) vs. 79.3% (88/111), χ2=5.34 and 10.41, both P<0.05]. Conclusions:Multiple pregnancies, HDP, and ART can increase the risk of SGA in VPIs. SGA infants may have increased risks of parenteral nutrition-related problems and EUGR during hospitalization. Symmetric SGA infants are more likely to develop EUGR.