This Beneficial Bacteria Is Linked To Better Fertility In Women Over 35 

When we talk about fertility as we get older, most of the attention goes to the ovaries. We hear about egg quality, ovarian reserve, hormones, and how all of those things begin to change with age.

But getting pregnant requires more than a healthy egg. Once an egg is fertilized, it still needs somewhere to successfully implant and grow.

That's where the endometrium comes in. The endometrium is the lining inside your uterus that thickens throughout your menstrual cycle in preparation for a potential pregnancy. If an embryo arrives, this is where it needs to implant.

And it turns out the health of that lining may be influenced by something you probably don't hear much about when talking about fertility: the bacteria living there.

Researchers studied 149 women between the ages of 20 and 45 who were undergoing fertility treatment. They collected samples from the endometrium and looked at the different bacteria living there, comparing women under 30, ages 30 to 35, and over 35.

One pattern quickly emerged: younger women had more Lactobacillus bacteria in their endometrium, and that abundance tended to decline with age. Interestingly, it wasn't that the entire uterine microbiome became dramatically different as women got older. Instead, researchers saw shifts in certain types of bacteria, particularly Lactobacillus.

They then followed the women for a year. During that time, 93 underwent embryo transfers, and 56 became clinically pregnant. While pregnancy rates weren't significantly different between the age groups, the women who became pregnant tended to have more Lactobacillus in their endometrium than those who didn't.

The researchers then wanted to know whether certain Lactobacillus species might be particularly helpful for the endometrium. That's where Lactobacillus gasseri came in.

When they tested several strains in the lab, L. gasseri stood out for its antioxidant and anti-inflammatory activity and its ability to adhere to endometrial cells. That made it a leading candidate for further testing, though the authors emphasize that it probably isn't the only beneficial strain. Instead, the findings point toward a Lactobacillus-rich uterine environment more broadly.

But the researchers took things one step further. They isolated a compound produced by L. gasseri called an exopolysaccharide, or EPS. This is considered a postbiotic, essentially a beneficial substance produced by bacteria rather than the live bacteria themselves.

This study doesn't show that taking a probiotic will improve fertility, and the researchers specifically caution against self-treating with over-the-counter L. gasseri products. The women in the study weren't given the postbiotic treatment. Researchers simply observed an association between their uterine bacteria and pregnancy outcomes, while the more experimental findings came from cells and mice.

There are other unanswered questions, too. Estrogen helps shape the reproductive tract microbiome, so it's possible that hormonal changes with age are influencing both Lactobacillus levels and the endometrium at the same time. Researchers can't yet say that losing these bacteria directly causes the endometrium to age or makes implantation more difficult.

Fertility after 35 is often framed almost entirely around the ovaries, but this study is a reminder that pregnancy depends on much more than producing a healthy egg.

The endometrium has to be ready, too.

Researchers still have a lot to learn about whether the uterine microbiome can be safely and effectively targeted in fertility treatment. But if future human studies confirm these findings, supporting the environment where implantation actually happens could become another important piece of reproductive care.