Why do I smell before my period?

One of the main reasons you might notice a change in smell before your period is hormone fluctuations.

Oestrogen levels drop before menstruation, and oestrogen plays a key role in maintaining vaginal acidity. This acidity is crucial for keeping good bacteria—primarily the lactobacillus genus—thriving. Lactobacillus species help maintain a low vaginal pH (around 3.5–4.5) by producing lactic acid as a byproduct of their metabolism. This acidic environment makes it harder for harmful bacteria to grow, keeping the vaginal microbiome balanced. Some Lactobacillus species, such as Lactobacillus crispatus, also produce hydrogen peroxide, which has antimicrobial properties and adds an extra layer of protection against pathogens.

Meanwhile, progesterone levels rise during the luteal phase (the phase before your period), which can impact immune responses and alter bacterial composition. Higher progesterone levels tend to favour anaerobic bacteria such as Gardnerella vaginalis over lactobacilli, which can influence changes in vaginal smell.

So, why does this matter?

The combination of lower oestrogen and higher progesterone before your period can lead to a less acidic vaginal environment, allowing bacterial imbalances to occur. This is why some people notice changes in their discharge or odour in the days leading up to their period.

Menstrual blood itself also plays a role. It has a metallic smell due to its iron content, and because blood is more alkaline, it can temporarily shift vaginal pH, further affecting odour.

Could it be an infection?

If your vaginal odour is particularly strong, fishy, or comes with unusual discharge or itching, it might be a sign of infection. Many people assume thrush is the most common culprit—I did too! But bacterial vaginosis (BV) is actually much more common and tends to flare just before or after your period due to pH disruptions.

Semen can also be a pH disruptor! The shift to a more alkaline environment makes it easier for BV-associated bacteria, like Gardnerella vaginalis, Prevotella, and Atopobium, to thrive. Symptoms of BV include a strong, fishy odour, increased discharge, and sometimes itching before or during your period.

Some women have a vaginal microbiome dominated by Lactobacillus iners, the most unstable of the Lactobacillus species. Unlike L. crispatus, which provides strong protection, L. iners produces weaker acids and fluctuates significantly throughout the menstrual cycle. It tends to be more dominant during lower oestrogen phases, such as around menstruation. L. iners is highly adaptable to changes in nutrient availability, allowing it to survive in both high- and low-oestrogen conditions. However, this adaptability comes at a cost—women with an L. iners-dominant microbiome may experience more pronounced symptoms before their period. As oestrogen drops and L. crispatus—one of the strongest acid producers—declines, the vaginal environment becomes less acidic, creating conditions where opportunistic bacteria can thrive. This can lead to an increased risk of BV-associated symptoms, including changes in discharge and odour.

Unlike L. crispatus, which is strongly protective, L. iners is more commonly found in women with BV and has even been described as a transitional species that can coexist with BV-associated bacteria like Gardnerella vaginalis. Some research suggests that L. iners can switch its behaviour depending on the environment, sometimes acting more like a pathogen than a protective microbe (1).

Recurring BV – what’s going on?

BV recurrence is incredibly common—about 80% of cases return within three months of antibiotic treatment (2). The issue with frequent antibiotic or over-the-counter treatments is that they can wipe out important microbes your vagina needs to stay balanced. That’s why it’s crucial to know what bacteria are present before treating it—different microbes require different treatments.

It’s also worth considering that BV-related bacteria can be shared with your sexual partner. If you’ve been treated but have unprotected sex, the bacteria you just got rid of might make their way back to you.

What can you do?

If you struggle with recurring BV, natural approaches can help. Supporting your vaginal microbiome with vitamin D and zinc can be a great place to start. Antimicrobials, probiotics, and pH-balancing products can also help maintain a healthy environment. But if BV keeps coming back, you should get checked out—it’s been linked to more serious reproductive health concerns like pelvic inflammatory disease, miscarriage, early pregnancy loss in IVF, preterm delivery, and postpartum complications such as endometritis and wound infections (3,4). BV also increases the risk of contracting STIs like HIV (5), HPV (6), gonorrhoea, and chlamydia (7).

Understanding your microbiome

Knowing your vaginal microbiome can help you get to the root of recurring issues. Companies like ScreenMe use next-generation sequencing to provide a detailed analysis of both vaginal and seminal microbiome health—so don’t just test yourself, test your partner too! If conventional treatments aren’t working, consider working with an intimate health practitioner for personalised advice.

References:

1. Tachedjian G, Ravel J, Forney LJ, et al. Lactobacillus iners: Friend or Foe? Trends in Microbiology. 2016;24(9):716-726. doi:10.1016/j.tim.2016.06.002.

2. Coudray M, Madhivanan P. Bacterial vaginosis—A brief synopsis of the literature. European Journal of Obstetrics & Gynecology and Reproductive Biology. 2020;245:143-148.

3. Lewis F, Bernstein K, Aral S. Vaginal Microbiome and Its Relationship to Behavior, Sexual Health, and Sexually Transmitted Diseases. Obstetrics & Gynecology. 2017;129(4):643-654.

4. Sobel J, Sobel R. Current and emerging pharmacotherapy for recurrent bacterial vaginosis. Expert Opinion on Pharmacotherapy. 2021;22(12):1593-1600.

5. Mehta S, Zhao D, Green S, Agingu W, Otieno F, Bhaumik R et al. The Microbiome Composition of a Man's Penis Predicts Incident Bacterial Vaginosis in His Female Sex Partner With High Accuracy. Frontiers in Cellular and Infection Microbiology. 2020;10.

6. Swidsinski A, Doerffel Y, Loening-Baucke V, Swidsinski S, Verstraelen H, Vaneechoutte M et al. Gardnerella Biofilm Involves Females and Males and Is Transmitted Sexually. Gynecologic and Obstetric Investigation. 2010;70(4):256-263.

7. Sanchez S, Garcia P, Thomas K, Catlin M, Holmes K. Intravaginal metronidazole gel versus metronidazole plus nystatin ovules for bacterial vaginosis: A randomized controlled trial. American Journal of Obstetrics and Gynecology. 2004;191(6):1898-1906.