Objective To determine the optimal timing of SARS-CoV-2 vaccination during pregnancy by investigating the transplacental transfer of SARS-CoV-2 antibodies from maternal SARS-CoV-2 vaccination or infection. Design Prospective cohort study Setting Samples were collected between June 6, 2022 and September 20, 2022 in Taiwan. Sample Seventy-five maternal-cord pairs, including 38 pairs with a primary maternal SARS-CoV-2 infection by the Omicron variant (BA.2.3.7) and 37 pairs without maternal infection. Methods SARS-CoV-2 antibodies against the nucleocapsid (anti-N), receptor-binding domain of the spike protein (anti-S), and the neutralizing antibody (nAb) titers against different SARS-CoV-2 variants were measured for the maternal-cord pairs. The participants were categorized based on the timing of their last vaccination, including during the third (T3) and second trimesters (T2) and before/during the first trimester (T1). Comparison of anti-spike protein antibody and neutralizing antibody concentrations was analyzed using the Kruskal-Wallis test followed by a post-hoc Mann-Whitney U test. Main Outcome Measures Anti-S concentrations of maternal and cord serum among T1, T2, and T3 groups. Results Among participants without SARS-CoV-2 infection, the highest anti-S levels ( p value for anti-S levels in maternal and cord plasma were <0.001 and <0.001, respectively) and highest nAb potency against both the Wuhan and Omicron strains ( p value for nAb against Omicron strain were both <0.001 for maternal and cord plasma) were observed in maternal and cord plasma in the T3 group. Conclusions A booster vaccine dose during the third trimester can provide maximum transplacental protection against the Wuhan wild-type strain and Omicron variant. Pregnant women are encouraged to receive vaccinations during pregnancy, ideally in the third trimester, for the highest level of neonatal protection.