Uterine microbiome tied to IVF pregnancy rate

Clinical takeaway: The uterine lining is emerging as a second front in age-related infertility, alongside ovarian reserve. Counseling can acknowledge that its microbiome may be a factor without endorsing OTC products.

When a patient over 35 asks why age is working against her, the answer she gets is almost always about eggs: ovarian reserve, AMH, antral follicle count, embryo quality. The uterus rarely comes up. Yet even with donor oocytes from young women, implantation rates fall and pregnancy loss rises with the recipient's age, which points to something about the lining rather than the egg.

Patients over 35 now make up a substantial share of US fertility care. IVF gives them real ovarian options, but when transfers keep failing with good-quality embryos then the workup runs out of places to look. Attention lands on the uterine lining next, and OTC products often get there first.

Vaginal and oral Lactobacillus products are marketed to support implantation, sold without strain standardization or dosing evidence. New evidence maps the bacterial shift in the aging uterine lining and ties it to who conceives after transfer.

Among women who went on to embryo transfer, endometrial Lactobacillus abundance separated those who conceived from those who did not. Women achieving clinical pregnancy carried more Lactobacillus overall and more L. crispatus specifically, while those who did not conceive carried more L. iners and Gardnerella vaginalis. Not every comparison reached significance, though several did.

Age tracked the same organisms in the same direction. Lactobacillus abundance was lower in women 30 and older than in those under 30, with no further decline between the 30-to-35 band and the 35-and-older band. What shifted was composition, not diversity: L. crispatus and L. gasseri fell with age while L. iners rose, and overall microbial diversity held steady across all three bands. Tissue from the oldest patients also expressed less HOXA10 and LIF, two proteins the lining produces when it is ready to accept an embryo, though this came from a 12-sample subset.

Clinical pregnancy per transfer did not differ by age, running between 57% and 65% across the three bands. With only 93 transfers total, the authors attribute that result to small numbers and to embryo selection. Within each age band, the microbiome pattern still separated conceivers from non-conceivers.

The interventional work stayed preclinical. Among five Lactobacillus strains isolated from healthy women, L. gasseri adhered best to human endometrial cells and ranked highest for antioxidant activity. Its purified exopolysaccharide (EPS) cut senescence in cultured human endometrial cells to 13.5% from 21.6%. Aged mice given daily intravaginal L. gasseri for 28 days showed less endometrial senescence and higher embryo implantation rates. Deleting the bacterium's EPS gene abolished the benefit, and adding EPS back restored it.

Endometrial tissue was biopsied at hysteroscopy from 149 women aged 20 to 45 being treated for infertility at a single center in Beijing, then sequenced and grouped by age: under 30, 30 to 35, and 35 and older. Transfer outcomes were tracked for 12 months. Because the sampling is cross-sectional, the age and pregnancy patterns are associations only, and the authors note that declining Lactobacillus may partly reflect falling estrogen rather than a microbial cause of its own.

What would move this is a human trial: a defined strain or purified postbiotic, a route shown to reach the endometrium, and implantation as the endpoint. None of that exists yet. Until it does, endometrial Lactobacillus is a research target rather than something to test or treat in practice.

"This suggests that a defined bacterial product, rather than only the living microorganism, may contribute to the observed effects on endometrial aging and receptivity, providing more feasible options for clinical intervention," said Rong Li, a physician at Peking University Third Hospital.

Source: Li M, et al (2026 Aug 6) Cell Host Microbe. Lactobacillus gasseri postbiotics ameliorate age-related decline in endometrial receptivity