Background: Eating carbohydrate foods with a high glycemic index (GI) has been postulated to result in fetoplacental overgrowth and higher infant body fat. A diet with a low glycemic index (LGI) has been shown to reduce birth percentiles and the ponderal index (PI). Objectives: We investigated whether offering LGI dietary advice at the first antenatal visit would result in a lower fetal birth weight, birth percentile, and PI than providing healthy eating (HE) advice. This advice had to be presented within the resources of routine antenatal care. Design: The Pregnancy and Glycemic Index Outcomes study was a 2-arm, parallel-design, randomized, controlled trial that compared the effects of LGI dietary advice with HE advice on pregnancy outcomes. Eligible volunteers who attended for routine antenatal care at <20 wk of gestation were randomly assigned to either group. Results: A total of 691 women were enrolled, and 576 women had final data considered. In the LGI group, the GI was reduced from a mean (±SEM) of 56 ± 0.3 at enrollment to 52 ± 0.3 (P < 0.001) at the final assessment. There were no significant differences in primary outcomes of fetal birth weight, birth percentile, or PI. In a multivariate regression analysis, the glycemic load was the only significant dietary predictor (P = 0.046) of primary outcomes but explained < 1% of all variation. Conclusion: A low-intensity dietary intervention with an LGI diet compared with an HE diet in pregnancy did not result in any significant differences in birth weight, fetal percentile, or PI. This trial was registered at https://www.anzctr.org.au as ACTRN12610000174088.