he requirement of leukocytes for initiation of term labor in women remains inconclusive. In general, resident and invading leukocytes produce the bulk of cytokines in cases of inlammation resulting from infection. Indeed, with infection, leukocytesmainly neutrophils, macrophages, and T lymphocytes-iniltrate the cervix, lower uterine segment, fundus, and membranes at the time of labor. Invading leukocytes and certain parenchymal cells produce cytokines and appear to be the primary source of myometrial cytokines (Young, 2002). By contrast, in the decidua, both stromal cells and leukocytes likely contribute. In the cervix, glandular and surface epithelial cells appear to produce cytokines.
he presence of cytokines in amnionic fluid and their association with preterm labor is well documented. But, their exact cellular origin-with or without recoverable microorganismsis not well deined. Amnionic fluid cytokines are most likely secreted by mononuclear phagocytes or neutrophils activated and recruited into the amnionic luid. Thus, the amount of amnionic luid IL-1 3 would be determined by the number of leukocytes recruited, their activational status, or the efect of amnionic fluid constituents on their IL-1 3 secretion rate.
Vaginal Microbiota. Factors that predispose to ascending infection and then preterm birth are a key focus of ongoing basic research. \1ucosal immunity and barrier function of the cervicovaginal epithelia, the microbiota composition in the vaginal tract, and their interplay are a major topic (Smith, 2017). From animal studies, mucosal immunity of the lower reproductive tract can be disrupted by viral infection with a subsequent enhanced susceptibility to ascending bacterial infection (Racicot, 2013, 2017). Along with the ability of the cervico vaginal epithelia to respond to environmental insults, the com position of the microbe ecosystem in the vaginal tract may also determine susceptibility to ascending infection.
that the nonpregnant vaginal tract hosts a complex microbial community (Gajer, 2012; White, 201l). Also described in
Chapter 65 (p. 1245), these community state types can difer widely among women who are all healthy. And, the vaginal microbiome changes during normal pregnancy (Aagaard, 2012;
Stout, 2017). Namely, the diversity and richness of microbe populations are reduced during pregnancy and become more stable. Compared with nonpregnant controls, Lactobacillus species show an enhanced dominance. Some but not all stud ies report an increased population of certain microbes-for example, Gardnerella vaginalis and Ureaplasma ureayticumqi in women with preterm birth (Donders, 2009; Nelson, 2014).
However, diferences in populations of pregnant women stud ied, in deinitions of preterm birth, and in data analysis may complicate interpretation of these data.
Some speciic microorganisms are detected more frequently than others in amnionic fluid of women with preterm labor (Gerber, 2003; Hillier, 1988; Yoon, 1998). hese include G vaginalis, Fusobacterium species, Mycoplasma hominis, and U ureayticum. Their identification was interpreted by some as presumptive evidence that speciic microorganisms are more commonly involved as pathogens in the induction of preterm labor. Another interpretation, however, is that given direct access to the membranes after cervical dilation, selected micro organisms, such as fusobacteria, are more capable of burrowing through these exposed tissues and will do so. Fusobacteria are found in the vaginal luid of only 9 percent of women but in 28 percent of positive amnionic luid cultures from pregnancies with preterm labor and intact membranes (Chaim, 1992).