The rate of exposure to second-hand smoke (SHS) is relatively high in several countries, including Vietnam, and health issues related to SHS have worsened in recent years, especially for pregnant women and their infants. Enhancement of knowledge, attitude, and practice (KAP) scores of pregnant women in Vietnam could raise practical interventions to protect their health and reduce complications of SHS. A cross-sectional study of 432 pregnant women who came to the Obstetrics Department of Bach Mai Hospital, Hanoi, Vietnam for antenatal care was conducted in 2016 to collect information about their KAP related to SHS. Composite mean scores from survey questions assessing their KAP were calculated on a 10-point scale, finding mean scores of 4.19, 7.45, and 4.30, respectively. Higher scores indicated better knowledge, attitude, and practice. Generalized linear models identified that age, occupation, living place, and sources of information were associated with SHS-related KAP. Findings from this study indicate that suitable programs related to SHS should be implemented to improve and reinforce health literacy to both mothers and smokers to reduce the harmfulness of smoking on women and their infants’ health.