Abstract: Objective To evaluate independent and combined effects of overweight/obesity and high-normal blood pressure during first trimester on the risk of gestational hypertension after 20 weeks of pregnancy（middle and late pregnancy）. Methods Using Tianjin Maternal and Child Health Information System, the basic information, body mass index（BMI）, blood pressure data in the first trimester and after 20 weeks of pregnancy of 34 913 pregnant women who received the first antenatal examination in six districts and Binhai New Area of Tianjin from 2009 to 2010 were collected and analyzed. Pregnant women were divided into emaciated, normal, overweight and obese groups according to their BMI in the first trimester, and into normal and high-normal blood pressure groups according to their blood pressure in the first trimester. Logistic regression analysis was used to analyze the independent influence of BMI grouping and blood pressure level in the first trimester on blood pressure after 20 weeks of pregnancy. According to the level of BMI and blood pressure in early pregnancy, the subjects were divided into four groups: BMI＜24.0 kg/m~2 with normal blood pressure, BMI≥24.0 kg/m~2 with normal blood pressure, BMI＜24.0 kg/m~2 with high-normal blood pressure, BMI≥24.0 kg/m~2 with high-normal blood pressure. Using BMI＜24.0 kg/m~2 and normal blood pressure group as the reference group, the combined effect of BMI and blood pressure level in the first trimester of pregnancy on blood pressure after 20 weeks of pregnancy was analyzed by logistic regression analysis and additive interaction model. Results Compared with normal BMI women, overweight/obese women had higher blood pressure after 20 weeks of pregnancy. Women with high-normal blood pressure had significant higher blood pressure than women with normal blood pressure at first trimester systolic blood pressure:（112.6±10.4） vs（105.7±9.9） mmHg, t=51.135, P＜0.001; diastolic blood pressure:（72.2±7.7） vs（67.5±7.0） mmHg, t=46.708, P＜0.001. Multivariate logistic regression analysis demonstrated that compared to women with normal BMI, overweight and obese women were at higher risk of having hypertension after 20 weeks of pregnancy（overweight: OR=2.090, 95%CI 1.643-2.659; obese: OR=4.473, 95%CI 3.400-5.883, respectively）. Compared to those with normal blood pressure, women having high-normal blood pressure during the first trimester had increased risk of gestational hypertension after 20 weeks of pregnancy（OR=4.465, 95%CI 3.604-5.532）. Multivariate logistic regression analysis showed that compared with the group with normal BMI and normal blood pressure in the first trimester, the risk of hypertension after 20 weeks of pregnancy in the group with normal BMI and high-normal blood pressure was 6.047（95%CI 4.467-8.185）, while which increased to 16.780（95%CI 12.684-22.198） in the group with overweight/obesity and high-normal blood pressure. The results of additive interaction showed that high-normal blood pressure and overweight/obesity in the first trimester had additive interaction on the occurrence of hypertension after 20 weeks of pregnancy. The relative excess risk of interaction（RERI） was 7.888（95%CI 4.346-11.430）, the attributable proportion of interaction（AP） was 0.470（95%CI 0.343-0.597）, and the synergy index（S） was 2.000（95%CI 1.535-2.605）. Conclusion Overweight/obesity and high-normal blood pressure in the first trimester are independent risk factors of gestational hypertension after 20 weeks of pregnancy, and there is an additive interaction between the two factors.