Fertility Facts: 15 Evidence-Based Insights on Age, Cycle, and Vaginal Health

While it’s common to think of conception mainly in terms of “egg meets sperm,” the reality is far more complex. Think of both male and female fertility as a puzzle: many different pieces need to fit together for conception to happen. 

Factors like age, whether or not your menstrual cycle is regular, and overall health certainly play a role, but so do lifestyle habits, medical conditions, and even the unique ecosystem of bacteria living in the vagina.

By looking at female fertility through this wider lens, it becomes clear that reproductive health isn’t defined by a single factor. Instead, it’s shaped by the interplay of biology, environment, and even lifestyle choices. Understanding how these pieces connect not only demystifies the fertility journey but also allows people with ovaries to make informed decisions and take proactive steps in supporting their reproductive potential. 

Key facts about fertility: what science tells us about getting pregnant

When it comes to getting pregnant, many people are surprised to learn how many different things need to align for conception to happen. Fertility isn’t just about timing sex; it’s influenced by age, lifestyle, overall health, and even the vaginal microbiome.

The vaginal microbiome, in particular, is now understood to be one of the most important yet overlooked players in fertility. A healthy microbiome can protect sperm, support implantation, and lower the risk of complications, while imbalances may silently interfere with conception.

Let’s explore 15 evidence-based facts about fertility that highlight just how multifactorial this journey can be.

1. Female fertility declines with age

Age is one of the strongest and most consistent predictors of fertility. People with ovaries are born with all the eggs they’ll ever have — around 1–2 million eggs when you're born, which drops to about 300,000 by puberty. From the mid-30s onward, egg quantity and quality decline more quickly, making it harder to conceive and increasing the risk of miscarriage.

Fertility starts to decline noticeably in your early 30s, with a sharper decline after 37 (although these are just averages). While many people still conceive in their late 30s and 40s, it may take longer or require support from a fertility specialist.

2. The fertile window lasts about six days in each menstrual cycle

Conception is only possible during a short window in each cycle, specifically, the days leading up to ovulation and the day of ovulation itself. Sperm can survive in the reproductive tract for up to five days under the right conditions, while the egg lives only 12–24 hours after release.

That means the fertile window is about six days long, but the highest chances of pregnancy come from having (unprotected) sex in the two days before you ovulate. Tracking your cycles, monitoring cervical mucus, and using ovulation predictor kits can help narrow this window, but even then, chance plays a big role.

3. About half of infertility cases involve male factors

Fertility challenges don’t only affect women, even though women often face the most scrutiny and societal pressure. In reality, male factor infertility contributes to up to 40–50% of infertility. Common issues include low sperm count, poor sperm motility (how well sperm swim), abnormal sperm shape, or combinations of these factors.

This highlights the importance of looking at fertility as a shared responsibility between partners. When couples are struggling to conceive, it’s just as critical to evaluate sperm health as it is to assess ovulation, menstrual cycles, or reproductive conditions. Male fertility can be influenced by lifestyle factors — like smoking, alcohol consumption, diet, and stress — as well as medical conditions, environmental exposures, and age.

Addressing male fertility early can make a significant difference. Treatments and interventions — ranging from lifestyle changes and supplements to medical therapies or assisted reproductive technology — can improve sperm quality and increase the chances of conception. Recognizing that fertility is a team effort helps remove stigma and ensures that both partners receive the care and guidance needed to maximize their chances of success.

4. Smoking, heavy drinking, and chronic stress lower fertility in both men and women

Lifestyle choices can have a direct impact on reproductive health. Smoking is linked to reduced egg quality, early menopause, and lower sperm counts. Heavy alcohol consumption can disrupt hormone regulation and impair sperm production. And chronic stress — whether emotional, physical, or financial — can interfere with ovulation and sperm health through hormonal pathways.

While it’s impossible to eliminate stress entirely, practicing stress management and cutting back on smoking and alcohol may help improve the chances of conception.

5. Certain conditions can reduce fertility

Certain medical conditions make getting pregnant more difficult, often because they interfere with ovulation, hormone balance, or the structure of the reproductive system.

Polycystic ovary syndrome (PCOS) is a common hormonal condition that affects up to 10% of people with ovaries during their reproductive years. It causes irregular ovulation, high levels of male hormones (androgens), and ovarian cysts, which can make it even more difficult to conceive.

Endometriosis involves tissue similar to the uterine lining growing outside the uterus, leading to inflammation and pain. It can block or damage fallopian tubes and hinder egg release, resulting in lower pregnancy rates.

Thyroid disorders, whether hypothyroidism or hyperthyroidism, can also disrupt fertility by affecting menstrual cycles and reproductive hormones, leading to irregular periods and an increased risk of miscarriage.

Fortunately, with early diagnosis and effective management — through medication, surgical procedures, or lifestyle changes — many people with these conditions can still conceive.

6. Birth control doesn’t affect your future fertility

A common misconception is that long-term birth control use makes it harder to conceive later on. Research shows this isn’t true: once you stop hormonal contraception, fertility generally returns fairly quickly, often within a few weeks to a few months, depending on the method.

For oral contraceptives (the pill), ovulation usually resumes within one to three months after stopping. Some people may notice irregular cycles right away, but this typically reflects an underlying condition rather than a lingering effect of the medication.

With hormonal IUDs and the implant, fertility tends to return very quickly once the device is removed, often within the first cycle. Non-hormonal (copper) IUDs don’t suppress ovulation at all, so removal is followed by an immediate return to baseline fertility.

The contraceptive shot (Depo-Provera) is the one exception that may take longer. Because the injection suppresses ovulation for several months at a time, it can take six to 12 months after the last shot for fertility to fully return.

It’s also important to remember that many people go on hormonal contraception not just to prevent pregnancy, but to manage conditions like PCOS, endometriosis, or irregular cycles, all of which can independently affect fertility. When birth control is stopped, those underlying conditions often reappear, and it can feel as though the contraception itself caused fertility issues when, in reality, the challenges stem from the medical condition being managed.

On top of that, most people come off birth control as adults, often in their late 20s, 30s, or 40s. Since fertility naturally declines with age, it’s easy to mistakenly blame contraception when the real culprit is time. What truly matters is the overall health of the reproductive system, not the number of years spent on contraception.

7. You can still get pregnant during your period 

Getting pregnant during your period is unlikely, but not impossible. Conception requires both a viable egg and viable sperm. Since sperm can survive inside the female reproductive tract for up to five days, sex toward the end of a period may still result in pregnancy if ovulation happens soon after bleeding stops. This is more likely for people with short or irregular cycles, where ovulation can occur earlier than the “day 14” average. Sometimes bleeding that looks like a period is actually spotting from ovulation or hormonal changes, which can also increase the chances of conception.

While menstrual bleeding and the hormonal state of the period phase usually create an environment less hospitable to sperm, variation in cycle timing and sperm survival means period sex isn’t foolproof protection against pregnancy. If you want to avoid pregnancy, reliable contraception is recommended even during your period. If you’re trying to conceive (TTC), the fertile window remains the most effective time to focus on.

8. Vaginal infections can lower fertility by disrupting the vaginal microbiome

A growing body of research shows that vaginal infections — particularly bacterial vaginosis (BV) and other forms of vaginal dysbiosis — can negatively impact fertility. These infections disrupt the balance of beneficial bacteria, especially Lactobacillus species, impair mucosal barrier function, and trigger local inflammation, all of which can interfere with sperm transport, fertilization, and implantation. Multiple prospective cohort studies and meta-analyses demonstrate that persistent or concurrent infections are associated with reduced fecundability and an increased risk of infertility, with stronger negative effects seen when infections are recurrent or long-lasting.

Infections in the genital area, like pelvic inflammatory disease or endometritis, can harm reproductive health. They can cause swelling in the uterus, trigger the immune system to react to sperm, and increase the chances of early pregnancy loss. These issues highlight how important it is to maintain a healthy vaginal microbiome for successful conception.

Not all vaginal infections affect fertility, though. BV and other bacterial imbalances can decrease the chances of getting pregnant. However, current evidence shows that yeast infections (vulvovaginal candidiasis) don’t seem to lower fertility. Studies show no significant difference in conception rates between women with yeast infections and those without.

9. A healthy vaginal microbiome dominated by Lactobacilli supports sperm survival and implantation

The vaginal microbiome is a delicate ecosystem of bacteria that plays a crucial role in fertility. In people of reproductive age, a healthy balance is typically dominated by Lactobacilli, such as L. crispatus and L. gasseri. These beneficial bacteria help maintain a low, acidic vaginal pH, produce antimicrobial substances like lactic acid and hydrogen peroxide, and reduce local inflammation. Together, these effects create an environment that supports sperm survival, fertilization, and implantation.

Lactobacilli also help maintain a healthy vaginal barrier by stopping bad bacteria from growing. This is important because disruptive bacteria can cause immune responses that harm sperm movement and the ability of the uterus to receive a fertilized egg. Studies show that women with a Lactobacillus-rich microbiome are more likely to have successful implantation and early pregnancy than those with less stable microbiomes. Meanwhile, reduced Lactobacillus and increased microbial diversity — or the presence of disruptive species like Gardnerella or Fannyhessea — have been linked to infertility, implantation failure, and adverse pregnancy outcomes.

Clinical studies suggest that restoring Lactobacillus dominance, including through probiotic therapy in women with dysbiosis-related infertility, can improve vaginal microbiome and pregnancy rates. While the benefits may be species-specific and influenced by individual factors, the current consensus is clear: a Lactobacillus-dominated vaginal microbiome creates the best environment for sperm survival and successful conception.

10. Untreated STIs like chlamydia and gonorrhea can cause infertility

Sexually transmitted infections (STIs) don’t always cause noticeable symptoms, which can make them easy to overlook. However, even asymptomatic infections can have long-term consequences for reproductive health. Chlamydia and gonorrhea, in particular, can lead to pelvic inflammatory disease (PID) if left untreated. PID is an infection of the reproductive organs that can cause scarring and blocked tubes, which can prevent eggs from meeting sperm. This increases the risk of infertility and also raises the likelihood of an ectopic pregnancy, where a fertilized egg implants outside the uterus.

Other STIs, like trichomoniasis or Mycoplasma genitalium, may also contribute to inflammation or imbalance in the vaginal microbiome, indirectly affecting fertility. Because these infections can be silent, regular STI screening is essential for anyone who is sexually active, and it becomes especially important for those trying to get pregnant. Early detection and treatment not only protect fertility but also support a healthier vaginal environment for conception.

11. Recurrent bacterial vaginosis is linked to higher IVF failure rates

Bacterial vaginosis is the most common vaginal infection in people of reproductive age, and it’s associated with a higher risk of female infertility and complications. Studies have shown that women undergoing in vitro fertilization (IVF) who also have recurrent BV are more likely to experience implantation failure or miscarriage.

This connection underscores just how important the vaginal microbiome is when it comes to reproductive outcomes.

12. Chronic vaginal inflammation can make it harder for sperm to reach the egg

Chronic vaginal inflammation can make it harder for sperm to reach the egg, which can be a challenge for those trying to conceive. This inflammation is often due to conditions like bacterial vaginosis or other forms of vaginal imbalances that disrupt the healthy bacteria known as Lactobacillus, allowing harmful bacteria to thrive.

When inflammation occurs, some bacteria can release toxins that may affect sperm function. This can lead to reduced motility, difficulties in capacitation (a crucial step for sperm to successfully fertilize the egg), and challenges in the fertilization process itself. A decrease in the protective Lactobacilli can also weaken the mucosal barrier and trigger immune responses, creating an environment that’s less friendly for sperm to thrive and reach the fallopian tubes.

Research shows that chronic inflammation in the genital tract is connected to lower pregnancy rates, decreased sperm quality, and fewer chances of conceiving. While much of the focus has been on bacterial vaginosis, it’s clear that additional studies are needed to fully understand how various causes of chronic vaginal inflammation may impact sperm transport and fertility.

13. A balanced vaginal pH can support conception

When it comes to fertility, one important thing that often gets overlooked is vaginal pH balance. Many people think that a more alkaline environment is better for sperm survival, and while that’s technically true, the best vaginal environment for conception is one that promotes a healthy vaginal environment, typically between 3.8 and 4.5. This is thanks to good bacteria, mainly the Lactobacillus species, which thrive in that environment. These friendly bacteria produce lactic acid, keeping the vaginal pH low and helping to protect against harmful organisms and any imbalances. A healthy, acidic environment usually means a well-functioning vaginal microbiota, reduced chances of infections, and better chances of getting pregnant.

On the flip side, when the vaginal pH becomes more alkaline (higher than 4.7), it’s associated with issues like bacterial vaginosis. This can lead to reduced fertility and even complications during pregnancy. 

It’s worth noting that when sperm enters, it can temporarily raise vaginal pH. However, what’s really important is that those Lactobacillus bacteria can quickly bring the pH back down to its optimal level. This quick reacidification is crucial for creating the best conditions for conception and for implantation to take place.

14. Fertility outcomes improve when vaginal microbiome imbalances are treated early

Research shows that fixing vaginal microbiome imbalances before trying to get pregnant can improve fertility, especially for women who have symptoms or are undergoing fertility treatments. These imbalances, often caused by bacterial vaginosis or other vaginal issues, happen when protective bacteria decrease and other bacteria increase. This change can make it harder for sperm to survive, reduce the chances of implantation, and increase the risk of early pregnancy loss.

Early action can help restore a healthy environment dominated by Lactobacilli, giving sperm and embryos the best chance to succeed. For bacterial vaginosis with symptoms, doctors recommend antibiotic therapy before trying to conceive. Some studies also suggest that probiotics may help rebalance the vaginal microbiome. In assisted reproduction settings like IVF, treating abnormal vaginal bacteria before embryo transfer is linked to higher pregnancy rates.

While there isn’t yet a standardized approach for screening or treating vaginal microbiome imbalances in women trying to conceive naturally, addressing issues early — especially if you have recurring symptoms — can improve reproductive outcomes. Personalized vaginal microbiome testing, like Evvy’s Vaginal Health Test, can identify hidden imbalances and guide steps to support a healthier vaginal environment, giving your fertility journey a stronger foundation.

15. Up to 30% of infertility cases are unexplained, but microbiome testing may reveal hidden causes

Despite major advances in reproductive medicine, up to one-third of infertility problems remain “unexplained,” meaning standard testing doesn’t reveal a clear medical reason why conception isn’t happening. For many people, this can be frustrating and discouraging, leaving them with little guidance on what to do next.

One reason for this gap may be that the vaginal microbiome has historically been overlooked by the medical community. Female reproductive health has long been underfunded and under-researched, so aspects of fertility that don’t fall neatly into hormone levels, egg count, or sperm health have received less attention. Only in recent years has research begun to reveal how critical the microbiome is for reproductive outcomes.

Emerging studies now show that imbalances in the vaginal and uterine microbiomes can affect sperm survival, fertilization, implantation, and early pregnancy, and may be a hidden factor in cases labeled as unexplained infertility. Testing the vaginal microbiome can uncover these subtle but important issues that traditional fertility assessments often miss, potentially revealing the missing piece of the puzzle for people struggling to conceive.

Supporting your fertility with Evvy

If you’re trying to conceive, it’s important to remember that fertility is affected by far more than just timing sex or stopping birth control. Age, medical conditions, lifestyle, and the balance of your vaginal microbiome all play a role in your reproductive health.

Evvy’s Vaginal Health Test helps identify hidden factors that could impair conception by providing personalized Fertility Insights. Evvy’s test measures not only the amount of good bacteria in your vaginal microbiome but also the presence of disruptive bacteria that research links to fertility challenges. The test provides your Microbiome Diversity Score, which reflects the number of different bacterial species in your vagina — fewer species generally indicate a healthier, more protective environment.

The test also reports on five specific bacteria associated with fertility outcomes: Fannyhassae, Gardnerella, Prevotella, Sneathia, and Ureaplasma. You can see what percentage of your microbiome each makes up and how your levels compare to optimal ranges based on the latest research.

For additional insight, the Expanded PCR Panel can screen for four of the most common STIs (trichomoniasis, gonorrhea, chlamydia, and Mycoplasma genitalium), which are known to impact fertility. By understanding both your protective and disruptive bacteria, along with potential infections, you can take informed, proactive steps to support reproductive health and give yourself the best chance of conceiving.