A 2026 study found that bacteria detected on the embryo transfer catheter were linked to lower live birth rates, likely reflecting the patient’s own microbiome being carried into the uterus during transfer.
Embryo transfer is a critical step in IVF. It involves loading an embryo into a small volume of media within a catheter, which is then passed through the vagina and cervix into the uterus, where the embryo is deposited.
The vagina and cervix are not sterile environments and contain a wide range of bacteria. As the catheter passes through this area, it can pick up these bacteria and potentially carry them into the uterus during transfer.
It’s not clear what impact this has on outcomes, but there is growing interest in the reproductive microbiome. Some bacteria, especially Lactobacillus species, have been associated with better outcomes, while others have been linked to lower pregnancy rates, although results across studies have been mixed.
A study by Boughanmi et al. (2026) looked at this by measuring bacteria in the vagina, cervix, and on the catheter tip after embryo transfer, and then related this to pregnancy outcomes. This gives us a snapshot of what bacteria may actually be carried into the uterus during embryo transfer.
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Study details
- Study type: Prospective study conducted between 2021 and 2023 at a single center in Tunisia
- Participants: 210 patients undergoing fresh or frozen embryo transfer (average age ~34.5). Patients were ≤42 years, had a normal uterus, doing their first or second transfer, with at least one good quality embryo.
- Embryo transfer: Fresh and frozen cycles, with both cleavage stage and blastocyst transfers; 1 or 2 embryos transferred.
- Contamination assessment: Samples were collected by swabbing the vagina, collecting cervical mucus, and cutting the catheter tip after transfer, which was then placed in a sterile container for bacterial culture or PCR. They defined a positive result as viable bacteria ≥10³ CFU/mL on the catheter, which is a reasonable threshold and sometimes used as a standard, diagnostic cutoff in microbiology (ie. for urinary tract infections).
- Primary outcomes: Pregnancy rate and live birth rate
Bacteria detected in the vagina, cervix, and catheter
The researchers collected samples from the vagina, cervix, and the catheter tip after embryo transfer to assess what bacteria were present and what may have been carried into the uterus during the procedure.
Vagina
- Vaginal culture was positive for bacteria in 79% of patients
- Bacterial vaginosis (BV) was identified in 12.4%
Cervix
- Cervical cultures were positive for bacteria in 53.8% of patients
Catheter
- Bacteria were detected on the catheter in 58.1% of patients when combining both bacterial detection methods (bacterial culture and PCR).
- In all cases, the same bacteria were also detected in the vagina and cervix, suggesting they originated from the patient’s own microbiota rather than from catheter handling before the procedure.
- Bacterial detection was 6.2x higher in patients with BV (88.5% vs 55.3%, odds ratio [95% CI]: 6.2 [1.8–21.4]). This may be related to the higher diversity of bacteria seen in BV.
- Bacterial detection was 3x higher in patients doing a fresh transfer. This may be due to factors in fresh cycles, like egg retrieval, hormone stimulation, and vaginal progesterone.
Lower live birth rates when bacteria were detected on the catheter
Patients with bacteria detected on the catheter had lower live birth rates compared to those without (10.0% vs 29.3%, p< 0.001).
Other analyses showed a similar pattern, with lower pregnancy and live birth rates when non-Lactobacillus bacteria were present on the catheter (such as Streptococcus, Actinomyces, and Gardnerella), and higher rates when Lactobacillus was detected.
Conclusion
This study found that bacteria were often detected on the catheter after embryo transfer, and this was linked to lower live birth rates. Bacteria were more common in patients with BV, while other analyses suggested better outcomes when Lactobacillus was present and worse outcomes with other bacteria.
It’s important to note that the catheter wasn’t contaminated beforehand. It likely picked up bacteria while passing through the vagina and cervix during transfer, meaning these results are due to the patient’s own microbiome being carried into the uterus, not a mistake during the procedure.
The difference in live birth rates was large, which is kind of surprising. This hasn’t been studied much, but a couple of other studies have also found lower pregnancy rates when bacteria are detected on the catheter, sometimes with similar differences. It’s possible this is a common process that happens during embryo transfer but hasn’t been explored in detail, so more research here is definitely needed.
One important point is how common this is. The study reported bacteria on the catheter in 58.1% of patients, but this combined two detection methods. When using the standard culture method (which detects viable bacteria), the rate was 33.8%. The difference in live birth rates was mainly seen using this method, and not with the more sensitive PCR method, although the study didn’t clearly report comparisons here.
Other research on the microbiome and IVF has been mixed, with concerns about reliability since the microbiome can change over time and is influenced by factors like the menstrual cycle, hormones, sexual activity, age, and ethnicity.
Overall, this study suggests the vaginal and cervical microbiome may influence IVF success, but it doesn’t show that testing or treating it will improve outcomes. The authors highlight BV as an important factor, and some guidelines already recommend screening for BV before IVF using a simple and inexpensive test called the Nugent score. BV is common, affecting about 23–29% of women, and most cases are asymptomatic or go unrecognized, although it can usually be treated with antibiotics (Muzny et al. 2020).
However, the study included a mix of patients and wasn’t designed to show cause and effect, so the size of this difference may be overstated. Importantly, this reflects the patient’s own microbiome rather than an issue with the clinic or procedure, and it’s not yet clear whether identifying or treating these differences would improve IVF outcomes.
Limitations: The study included a mix of fresh and frozen transfers, different embryo stages, and both single and double embryo transfers. These factors weren’t tightly controlled and could affect the results. There was also no group where steps were taken to reduce bacterial transfer, like cleaning the vagina and cervix before transfer.
Want to read more about the microbiome and IVF?
This post reviews what research currently shows about the vaginal and endometrial microbiome and how it may relate to fertility and IVF outcomes. While Lactobacillus species are often considered beneficial and are commonly found in the reproductive tract, studies examining whether microbiome differences actually affect IVF success have produced mixed results. Read more.
Researchers in a 2023 study found that probiotic supplementation had no impact on positive hCG pregnancy rates, but may improve miscarriage rates. Read more.
A 2025 study found that RIF and RPL patients with abnormal EMMA & ALICE results that had antibiotic and/or probiotic treatment showed similar pregnancy rates to those with normal EMMA & ALICE results. Read more.
Reference
About Embryoman
Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.