Figure 2.

Microbial characteristics in the FRTA. Changes in hormones and microbiome before and during pregnancy. As the follicular phase begins, the levels of estrogen and progesterone rise, though the peaks of the two important hormones occur at different stages. Fluctuations in the hormones lead to changes in the vaginal environment and microbiota: an increase in the abundance of Lactobacillus spp. and a decrease in the vaginal pH and microbial diversity, which gradually makes the vaginal context more favorable for conception. It should be noted that the correct order of a complete menstrual cycle is the follicular phase, ovulation, and the luteal phase. B. The role of Lactobacillus on fertility. The decrease of Lactobacillus is associated with lower lactic acid and H₂O₂ levels in the vagina, which further leads to the rise of environmental pH and overgrowth of pathogenic bacteria. The expansion of pathogenic bacteria in the vagina often induces BV and infertility. C. Five CSTs in the vagina. Each CST is dominated by different strains. Among them, four CSTs (CST I, CST II, CST III, and CST V) are dominated by Lactobacillus spp., while CST IV consists of a diverse microbial community. D. Ethnic variations of Lactobacillus proportions in the vagina based on limited studies. The vaginal microbiota of most healthy Caucasian women from North America is dominated by Lactobacillus spp. [111,117], while Asian and black women appear to have a lower proportion of Lactobacillus spp. [111,117]. Nevertheless, further confirmation of this observation is still needed. FRT, female reproductive tract; BV, bacterial vaginosis; CST, community state type.