The rate of vertical transmission of Ureaplasma spp. is 45% to 55% in full-term and 58% in preterm infants. Similar data are lacking for M. hominis. Vertical transmission of mycoplasmas occurs in utero or during delivery. In utero transmission occurs either transplacentally or by an ascending route from a colonized maternal genital tract. Mycoplasmas have been isolated from maternal blood at the time of delivery and from amniotic fluid, endometrium, placenta, and aborted fetal tissue. Mycoplasmas also have been isolated from the mucosal surfaces of newborn infants delivered by cesarean section performed before the onset of labor and rupture of amniotic membranes. In utero transmission is thought to be more common among preterm infants, whereas the majority of full-term newborns acquire mycoplasmas at delivery through contact with a colonized birth canal. Colonization of newborn infants is increased in the presence of chorioamnionitis and with decreasing
gestational age and birth weight, and it is highest among infants with a birth weight of less than 1,000 g. Postpartum or nosocomial transmission in neonates is not well documented, but probably occurs.

The role of these organisms in neonatal disease continues to be investigated and defined. Sufficient evidence exists to implicate both organisms as true neonatal pathogens. Mycoplasma hominis and Ureaplasma spp. have been recovered from the lungs, brain, heart, and viscera of aborted fetuses and stillborn infants with histologic finding of bronchopneumonia. The genital mycoplasmas also have been isolated from blood, urine, cerebrospinal fluid (CSF), and lung tissue of newborn infants with clinical signs of infection. The following clinical associations with Ureaplasma spp. have been made: fatal neonatal pneumonia in a term infant documented by isolation of the organism from lung at autopsy and demonstration of elevated serum IgG and IgM titers to Ureaplasma spp. in the infant; pneumonia and persistent pulmonary hypertension in five infants from whom Ureaplasma spp. was isolated from blood, endotracheal aspirate, pleural fluid, or lung at autopsy; afebrile pneumonitis in infants younger than 3 months; chronic lung disease in low-birth-weight infants whose respiratory tracts were colonized with Ureaplasma spp. in the first week of life; chronic lung disease in four infants in whom Ureaplasma spp. was recovered from lung biopsy tissue; meningitis in both preterm and full-term infants; osteomyelitis of the femur; nonimmune hydrops fetalis; and scalp abscess at the site of a fetal scalp electrode.