Objective:To investigate the effect of the colonization of group B streptococcus (GBS) in pregnant women and the use of intrapartum antibiotic prophylaxis (IAP) on perinatal outcomes.Methods:The clinical data of 1 078 pregnant women and their newborn babies delivered in Tongzhou and Xicheng hospital areas of Beijing Friendship Hospital from January 2020 to February 2021 were analyzed retrospectively. The clinical characteristics, complications, perinatal outcomes and neonatal prognosis of the pregnant women with positive and negative GBS were compared according to the GBS colonization, and the effects of IAP on the clinical characteristics, complications, perinatal outcomes and neonatal prognosis of the pregnant women with positive GBS was compared. The measurement data with normal distribution is expressed by x±s, two independent sample t-test was used for comparison between groups. Counting data were expressed in cases or cases (%), Inter group comparison χ 2 test. Results:Among 1 078 pregnant women, 93 were GBS positive (8.6%, GBS positive group), and 93 cases matching the clinical data of the GBS positive group were selected as the GBS negative group. There was no significant difference in age, gestational age, number of pregnancies, number of births, mode of delivery, abnormal blood pressure, abnormal blood glucose, abnormal thyroid function and the proportion of any complication in the GBS positive group (P values were 0.630, 0.613, 0.311, 0.761, 0.163, 0.601, 0.467, 0.388, 1.000, respectively). The proportion of neonates in GBS positive group transferred to pediatrics, the incidence of neonatal infection, pneumonia, anemia and bacterial infection rate were higher than those in GBS negative group (57.0%(53/93) vs 23.7%(22/93), 10.8%(10/93) vs 3.2%(3/93), 29.0%(27/93) vs 14.0%(13/93), 21.5%(20/93) vs 8.6%(8/93), 22.6%(21/93) vs 6.5%(6/93)) (χ 2 values were 21.47, 4.05, 6.24, 6.05, 9.75, respectively; the P values were <0.001, 0.044, 0.012, 0.014, 0.002, respectively). The hospitalization time of neonates transferred to pediatrics was longer than that of GBS negative group (4(0,5) d vs 0(0,0) d, Z=-4.03, P<0.001). The proportion of neonates in IAP group transferred to pediatrics and the incidence of pathological jaundice and adverse outcomes were lower than those in non IAP group (51.1% (24/47) vs 78.1% (25/32), 44.7% (21/47) vs 78.1% (25/32), 42.6% (20/47) vs 65.6% (21/32)) (χ 2 values are 5.92, 8.76 and 4.06, respectively, P values are 0.015, 0.003 and 0.044 respectively). The hospitalization time of neonates transferred to pediatrics was shorter than that of non IAP group (3(0,5) d vs 5(2,7) d, Z=-2.60, P=0.009). There was no significant difference between the two groups in terms of delivery mode, water breaking time, birth weight of neonates, infection, pneumonia, bacteremia/sepsis, and anemia ( P values were 0.073, 0.085, 0.479, 0.538, 0.157, 0.161, 0.238). Conclusions:GBS colonization in the lower genital tract does not increase the incidence of intrauterine infection, premature rupture of membranes and spontaneous preterm delivery in pregnant women, but the risk of bacterial infection and adverse outcomes in newborns is higher, the rate of paediatric transfer is higher, and the hospital stay is longer. IAP can reduce the incidence of neonatal pathological jaundice and shorten the hospital stay.