Considering the 20-30% increase in risk for fetal mortality, pregnancy-related Listeria monocytogenes infections present a unique public health challenge. Listeriosis outbreaks greatly vary in terms of associated pregnancy-related cases per outbreak. Currently, there are no validated tests that help predict which L. monocytogenes strains are likely to cause adverse effects in pregnancy. A 2011 listeriosis outbreak in the US from contaminated cantaloupes was deadly in nonpregnant older adults but caused few pregnancy-related cases. The five 2011 L. monocytogenes outbreak strains were compared in terms of in vitro invasiveness (individually and in mixtures) and biofilm formation to a known primate stillbirth isolate 12443 in human cell lines. Caco2, C3A and BeWo cell lines representing the three important barriers viz., gastrointestinal epithelium, liver and the placenta, respectively, were used in the invasion assays. These barriers need to be crossed by L. monocytogenes in order to infect the fetus and result in adverse effects during pregnancy. Relative to a stillbirth L. monocytogenes isolate, the 2011 strains appeared to be less invasive as mixtures in a human placental cell line (BeWo) than liver- and gastrointestinal-based cell lines. Our results suggest that relative to a known L. monocytogenes stillbirth isolate, this reduced invasive capacity of the 2011 strains, especially in mixtures, may have led to fewer pregnancy-related cases. Additionally, the 2011 strains also formed less biofilm than the stillbirth isolate. Our results suggest that in vitro invasiveness in relevant human cell lines and biofilm formation could help predict the likelihood of L. monocytogenes strains to cause adverse effects in pregnancy.