Studies have shown that maternal blood pressure level is associated with neonatal birthweight, but the results are not exactly consistent. As the most common hypertensive disorders during pregnancy, the mechanism of gestational hypertension and pre-eclampsia that affect fetal growth remain unclear. Our objective was to examine the association of gestational hypertension and pre-eclampsia with the risk of low birthweight (LBW) and small-for-gestational-age (SGA). Data were obtained from the China-US Collaborative Project for Neural Tube Defects Prevention, a large population-based cohort study. We selected participants who were registered in two southern provinces, had exact information on gestational blood pressure and pregnancy outcomes, and were not affected by chronic hypertension. Logistic regression was used to adjust for the effects of the main potential confounders, including age, body mass index, education, occupation, ethnicity, folic acid use, and parity. The overall incidences of LBW and SGA were 2.25% and 5.86%, respectively. The incidences of LBW/SGA were 3.58%/7.58% and 6.02%/10.67% for gestational hypertension and pre-eclampsia group, relative to 2.11%/5.68% and 2.16%/5.74% for normal group. The adjusted odds ratios associated with gestational hypertension/pre-eclampsia were 1.77 (95% CI: 1.63, 1.92)/3.01 (95% CI: 2.67, 3.40) for LBW and 1.40 (95% CI: 1.32, 1.48)/2.02 (95% CI: 1.84, 2.22) for SGA, respectively. The early onset of gestational hypertension/pre-eclampsia appeared to be a relatively more detrimental exposure window for both LBW and SGA. Our results support an association between gestational hypertension or pre-eclampsia and the increased risk of LBW and SGA.