**1. Introduction**

Until the beginning of the twentieth century, it was believed that the fetus came into the world free of microbes. Today, there are numerous indications that disprove this hypothesis and suggest that there is already a presence of microbes in the fetal-placental

unit. A useful model for examining the causes of numerous disorders and the development of human health is the neonatal microbiome. Multiple studies have described the importance of the first 1000 days of life when basic physiological exposures to microbes can have profound effects on future health. The establishment of a healthy microbiome, comprising trillions of microbial cells and their respective genomes, is critical during this period [1–3]. The neonatal microbiome begins as early as the prenatal period, where many studies suggest that the maternal microbiota colonizes the infant through the *in utero* environment. From here, mode of delivery, breastfeeding, diet, and environment lead to a rapid expansion of bacterial complexity as the microbiome becomes established. This stable trajectory of progressive bacterial colonization becomes critical for immune priming, normal function in multiple organ systems, and lays the foundation for future health. The mode of delivery, whether by cesarean section or vaginal delivery, can influence the neonatal immune system and subsequent susceptibility to infections. Identifying vaginal infections that are dangerous to pregnancy and the fetus from those that are bothersome but harmless and manageable, as well as their precise management, is a challenge for medical experts [4, 5].

The following chapter is the result of a literature review in order to investigate the effects of vaginitis on pregnancy outcomes. This refers to an investigation of the normal microflora of the vagina, its defense mechanisms, and the microorganisms that colonize it. The second part refers to the pathological colonizations of the vagina, the types of vaginitis that cause them, and their diagnosis. Next, the changes that the vagina undergoes during pregnancy, its pathology from the colonization of pathogenic microorganisms, and their treatment are described. Finally, the effects of vaginitis on the outcome of pregnancy are described, with pathologies such as chorioamnionitis, premature birth, and spontaneous abortion.

#### **1.1 Background and context**

In recent years, vaginal seeding has drawn interest as a potential strategy to improve the development of the neonatal microbiome. The community of microbes that live in the human body, notably the gut, skin, and mucous membranes, is referred to as the microbiome. It is essential for the development of the immune system, metabolism, and defense against pathogens, among other elements of health. The newborn is exposed to a wide variety of germs in the birth canal during a vaginal birth. This exposure is believed to contribute to the initial colonization of the infant's gut with beneficial bacteria, such as Bifidobacterium and Lactobacillus species. These bacteria are associated with immune system development, regulation of inflammation, and protection against certain diseases.

Cesarean section (C-section) delivery, however, bypasses the vaginal birth canal, resulting in a different microbial exposure for the newborn. Infants born *via* C-section tend to have a microbiome composition that differs from those born vaginally. Research has shown that these differences may have implications for the infant's health as C-section-born infants have been found to have an increased risk of certain health conditions, such as allergies, asthma, atopic disease, obesity, and autoimmune disorders [3–5].

Vaginal seeding is proposed as a method to mimic the microbial exposure of a vaginal birth for infants born *via* C-section. It involves the collection of maternal vaginal fluids, typically using a sterile swab, which is then transferred onto the newborn's skin or mouth within the first moments after birth. The goal is to introduce the mother's vaginal microbiota to the newborn, potentially promoting a microbial colonization pattern
