Introduction: Sexually transmitted infections caused by Chlamydia trachomatis and Mycoplasma genitalium can have significant implications during early childhood. This study aimed to assess maternal antibodies to C. trachomatis and M. genitalium in newborns, their vanishing, and offspring's own seroconversion to these pathogens during the first 3 years of life.

Material and methods: Altogether, 309 mother-neonate pairs originally enrolled in the prospective Finnish Family HPV (FFHPV) cohort study at Turku University Hospital, Finland, were analyzed for serum IgG antibodies to plasmid protein gene 3 (pGP3) for C. trachomatis and M. genitalium protein of adhesion (MgPa N-term) and recombinant MgPa for M. genitalium using multiplex serology, by serial sampling during a 3-year follow-up.

Results: A significant correlation between maternal and neonate antibodies to both C. trachomatis and M. genitalium was evident up to 2 months after birth and to C. trachomatis also at 6 months (p < 0.001). During the first 3 years of life, three children seroconverted IgG antibodies to C. trachomatis and one to M. genitalium. At the last (36-month) follow-up visit, five (2.1%) children were seropositive for C. trachomatis and only one (0.4%) for M. genitalium.

Conclusions: Both C. trachomatis and M. genitalium IgG antibodies are transferred from the mother to her offspring during pregnancy; similarly, this is shown for nearly all maternal IgG antibodies. Seroconversion for both C. trachomatis and M. genitalium in early childhood was a rare event. Further studies are required to elucidate the significance of C. trachomatis and M. genitalium antibodies acquired in early life.