The association between the pre-pregnancy vaginal microbiome and time-to-pregnancy: a Chinese pregnancy-planning cohort study

Abstract

Background: Although sexually transmitted infections are regarded as the main cause of tubal infertility, the association between the common vaginal microbiome and female fecundability has yet to be determined. The objective of this study was to find convincing evidence relating to the impact of the vaginal bacterial structure on the fecundability of women planning pregnancy.

Methods: We recruited women who took part in the Free Pre-pregnancy Health Examination Project from 13 June 2018 to 31 October 2018 (n = 89, phase I) and from 1 November 2018 to 30 May 2020 (n = 389, phase II). We collected pre-pregnancy vaginal swabs from each subject; then, we followed up each subject to acquire the pregnancy-planning outcome in 1 year. In phase I, 16S rRNA gene sequencing was performed to investigate the vaginal bacterial content between the pregnancy and non-pregnancy groups. These findings were verified in phase II by applying a quantitative real-time polymerase chain reaction for the measurement of the absolute abundance of specific species. Cox models were used to estimate fecundability ratios (FR) for each vaginal microbiome type.

Results: In phase I, 59.6% (53/89) of women became pregnant within 1 year. The principal coordinate analysis showed that the pre-pregnancy vaginal microbial community structures of the pregnant and non-pregnant groups were significantly different (PERMANOVA test, R² = 0.025, P = 0.049). The abundance of the genus Lactobacillus in the pregnancy group was higher than that of the non-pregnant group (linear discriminant analysis effect size (LDA) > 4.0). The abundance of the genus Gardnerella in the non-pregnant group was higher than those in the pregnant group (LDA > 4.0). In phase II, female fecundability increased with higher absolute loads of Lactobacillus gasseri (quartile Q4 vs Q1, FR = 1.71, 95%CI 1.02-2.87) but decreased with higher absolute loads of Fannyhessea vaginae (Q4 vs Q1, FR = 0.62, 95%CI 0.38-1.00). Clustering analysis showed that the vaginal microbiome of type D (characterized by a higher abundance of Lactobacillus iners, a lower abundance of Lactobacillus crispatus and Lactobacillus gassri) was associated with a 55% reduction of fecundability (FR = 0.45, 95%CI 0.26-0.76) compared with type A (featuring three Lactobacillus species, low Gardnerella vaginalis and Fannyhessea vaginae abundance).

Conclusions: This cohort study demonstrated an association between the pre-pregnancy vaginal microbiome and female fecundability. A vaginal microbiome characterized by a higher abundance of L. iners and lower abundances of L. crispatus and L. gasseri appeared to be associated with a lower fecundability. Further research now needs to confirm whether manipulation of the vaginal microenvironment might improve human fecundability.

Keywords: Cohort study; Fecundability; Lactobacillus; Time-to-pregnancy; Vaginal microbiome.

Fig. 1

Study design and participants

Fig. 2

Differences in the vaginal microbiome with different pregnancy outcomes. A, B PCoA analysis based on Jaccard distance: PC 1, PC2, and PC3 could explain 33.33%, 9.07%, and 8.58% of the variation, respectively. C Lefse analysis; the threshold of LDA value was 4.0. D The rank of the Gini index from the random forest model. PCoA, principal coordinate analysis; Lefse, linear discriminant analysis effect size

Fig. 3

Scatter diagrams showing the relative abundances of the genera between the pregnant and non-pregnant groups. The middle lines represent the median, while the error bars represent the interquartile range. P values were acquired by the Kruskal-Wallis test

Fig. 4

Types of vaginal microbiome and fecundability. A Clustering analysis based on the absolute loads of five bacterial species, based on the z-score of the log₁₀(absolute load). B Kaplan-Meier plots for the cumulative pregnancy rate across different vaginal microbiome types