Background: Evidence from previous studies suggest that Lassa fever, a viral haemorrhagic fever endemic to West Africa has high case fatalities, particularly in pregnancy. While there have been remarkable innovations in vaccine development, with some Lassa vaccines undergoing early clinical trials. An understanding of Lassa antibody kinetics and immune responses will support vaccine design and development. However, there is currently no evidence on the antibody kinetics of Lassa (LASV) in pregnancy. Our study sought to estimate the efficiency of transplacental transfer of LASV IgG antibodies from the mother to the child. Methodology/Principal findings: The study made use of data from a prospective hospital-based cohort of pregnant women enrolled at the antenatal clinic and followed up at delivery between February and December 2019. Blood samples from mother-child pairs were evaluated for antibodies against Lassa virus. The study demonstrates a transplacental transfer of LASV IgG of 75.3% [60.0–94.0%], with a significant positive correlation between maternal and cord concentrations and a good level of agreement. The study also suggests that transfer may be more variable in women with 'de novo' antibodies compared to those with pre-existing antibodies. Conclusions/Significance: The study shows that maternal antibody levels play an important role in determining transfer efficiency of Lassa antibodies to the new-born; and while the evidence is preliminary, the study also suggests that transfer efficiency may be less stable in acute or recent infection, as such timing of vaccination before pregnancy, that is in women of childbearing age may be more appropriate for protection of both pregnant women and their neonates. Author summary: Lassa fever, an epidemic-prone viral infection which frequently occurs in West African countries has a high mortality in pregnant women and their new-borns. Vaccination is an ideal method for preventing infection and while there are currently no licensed vaccines for Lassa fever, there have been marked innovations in Lassa vaccine development with some vaccines currently in early trials. Vaccine development requires background information on disease prevalence, incidence, and antibody kinetics. Our study sought to understand the kinetics of Lassa antibodies in pregnant women and their new-borns. This is the first study to evaluate the transfer of Lassa antibodies between the mother and child. We found that maternal antibody levels play an important role in determining how well antibodies are transferred from the mother to the child and may be influenced by the time when antibodies developed, with transfer of antibodies being less stable in those with recent infection. This suggest that for protection of both pregnant women and their neonates, it may be more appropriate to vaccinate women before they become pregnant, that is women of child-bearing age. While, the evidence is not conclusive, it is an important first step to understanding the kinetics of Lassa antibodies. [ABSTRACT FROM AUTHOR]