Relationship between vaginal group B streptococcus colonization in the early stage of pregnancy and preterm birth: a retrospective cohort study oleh Sho Tano, Takuji Ueno, Michinori Mayama, Takuma Yamada, Takehiko Takeda, Kaname Uno, Masato Yoshihara, Mayu Ukai, Teppei Suzuki, Yasuyuki Kishigami, Hidenori Oguchi