Objective To investigate the correlation of ureaplasma urealyticum(UU) and chlamydia trachomatis(CT) infections in the second trimester of pregnancy with small for gestational age (SGA)infants, in order to provide reference for early prevention of adverse pregnancy outcomes. Methods From May 2018 to May 2020, 112 parturients with SGA infants born in the Beijing Huairou Maternal and Child Health Care Hospital were selected as the observation group, and 112 parturients with normal newborns born at the same period were selected into the control group. Real-time fluorescent quantitative PCR (qPCR) method was used for CT and UU detection. Logistic regression was used to analyze the factors affecting the occurrence of SGA. Results The maternal UU infection rate(40.18%), CT infection rate(24.11%) and UU+CT infection rate(15.18%) of the observation group were higher than those of the control group (7.14%, 4.46%, 2.68%) with significant differences (χ²= 33.836, 17.646, 9.278, P<0.05). The proportions of fetal distress, neonatal asphyxia, admission to neonatal ward, premature delivery, premature rupture of membranes and early-onset sepsis in the observation group were significantly higher than those in the control group (χ²= 4.074, 4.573, 8.518, 9.475, 7.146, 4.167, P<0.05). Multivariate Logistic regression analysis showed that UU infection and CT infection were independent risk factors affecting the occurrence of SGA (OR=2.674, 95%CI:1.149 - 6.223; OR=2.762, 1.139 - 6.698, P<0.05). Conclusions UU and CT infections in the second trimester are related to SGA. Early screening of UU and CT for women of childbearing age and pregnant women has important clinical significance.