Only partially understood host defense mechanisms operate against infections affecting maternal and fetal morbidity. Subclinical ascending infections through the lower female genital tract are predominant worldwide. Important micronutrient deficiencies may prevail in low-income countries where these infections are much more common than in high-income countries. Important morbidities related to poor perinatal outcome both for the mother and for the fetus and newborn comprise preterm birth, prelabor rupture of membranes, placental abruption (predelivery detachment of the placenta), postpartum sepsis and maternal anemia. In the fetus, sepsis and intrauterine growth retardation are suspected to be consequences of ascending maternal infections. In the newborn, septicemia and respiratory disorders as well as some neurological disorders seem to be consequences of such ascending genital infections in the pregnant woman. It is concluded that much more attention should be given to efforts to elucidate the host defense mechanisms and antimicrobial barriers from the vagina through the cervix, fetal membranes and amniotic fluid including the early fetal immunocompetence in the second and the third trimester of pregnancy.