A woman’s fertility depends on several systems working together: a healthy supply of eggs, hormones that trigger ovulation on schedule, open fallopian tubes, a receptive uterine lining, and cervical fluid that helps sperm reach the egg. When all of these align, the chance of conceiving in any given cycle is roughly 20 to 25 percent for women in their early to mid-twenties. That number holds relatively steady through the early thirties, then drops more sharply after 37.
Egg Supply and Why It Matters
Every woman is born with a finite number of eggs, around 2 million. By puberty, that count has already fallen to about 400,000. Only about 400 to 500 of those will ever be released through ovulation over a lifetime. The rest gradually break down in a natural process that accelerates with age. By age 37, roughly 25,000 remain.
This reserve isn’t just about quantity. Egg quality, meaning the genetic integrity of each egg, also declines over time. Older eggs are more likely to have chromosomal errors, which raises the risk of failed implantation, miscarriage, or genetic conditions. This is the single biggest reason fertility drops with age, and it’s something no lifestyle change can fully reverse.
How Hormones Drive Ovulation
Fertility hinges on a monthly hormonal chain reaction. Early in each cycle, the brain releases a hormone that stimulates a group of follicles in the ovaries. One follicle becomes dominant and produces rising levels of estrogen, which eventually triggers a surge of luteinizing hormone (LH). That LH surge is the direct signal for ovulation: the egg is released about 36 hours after the surge begins and roughly 10 to 12 hours after the surge peaks.
After the egg is released, the empty follicle produces progesterone, which prepares the uterine lining for a potential pregnancy. If the hormonal signals at any point in this chain are too weak, mistimed, or absent, ovulation may not happen at all. Conditions like polycystic ovary syndrome, thyroid disorders, and high stress levels can all disrupt this process.
Two blood markers give a snapshot of where things stand. AMH (anti-Müllerian hormone) reflects the remaining egg supply: levels between 1.0 and 3.0 ng/mL are considered average, while anything under 1.0 is low. FSH, tested on day 3 of the cycle, is another signal. Levels under 10 are generally reassuring, but when the ratio of FSH to LH climbs too high, it can indicate diminished reserve even when individual numbers look normal.
The Fertile Window and Cervical Mucus
Sperm can survive inside the fallopian tubes for 3 to 5 days, but the egg lives only 12 to 24 hours after ovulation. That means the fertile window is about six days long: the five days before ovulation and the day of ovulation itself. Timing intercourse within this window is one of the most basic requirements for conception.
Cervical mucus plays a surprisingly active role. For most of the cycle, it forms a thick barrier that blocks sperm. As ovulation approaches, rising estrogen transforms it into a clear, stretchy, slippery fluid often compared to raw egg whites. This fertile-quality mucus creates channels that help sperm swim through the cervix, into the uterus, and up to the fallopian tubes. Women who produce little of this mucus, whether from hormonal issues, medication side effects, or other causes, may have a harder time conceiving even if everything else is working well.
Fallopian Tubes: More Than a Passageway
The fallopian tubes are where fertilization actually happens. Tiny hair-like structures called cilia line the inside of each tube, creating wave-like motions that guide the egg from the ovary toward the uterus. But the tubes do far more than transport. They provide the environment where sperm undergo final maturation, where the egg is fertilized, and where the resulting embryo spends its first four days of development before reaching the uterus.
Tubal problems account for up to one third of female fertility issues. Infections (particularly from chlamydia or gonorrhea), endometriosis, and prior surgeries can damage the delicate inner lining or block the tubes entirely. Even partial damage that doesn’t fully block the tube can impair its ability to pick up eggs or nurture an early embryo.
The Uterine Lining and Implantation
Once an embryo reaches the uterus, it needs a thick, nutrient-rich lining to implant into. Each cycle, estrogen and progesterone build up this lining in preparation. Research on IVF outcomes shows that a lining thicker than 10 to 12 mm is associated with the highest live birth rates, while anything below 6 mm dramatically reduces the chance of a successful pregnancy. The lining’s thickness and quality depend heavily on adequate hormone levels and good blood flow to the uterus.
How Age Changes the Picture
Age is the strongest single predictor of female fertility. A large North American study tracking couples trying to conceive found that the per-cycle chance of pregnancy stays relatively stable from the early twenties through age 33. After that, fertility begins to slide, with a noticeable drop after 37. Women aged 40 to 45 were 60% less likely to conceive in any given cycle compared to women aged 21 to 24.
The cumulative numbers tell the story clearly. After 12 months of trying, about 79% of women aged 25 to 27 had conceived, compared to roughly 75% of women aged 34 to 36, 67% of women aged 37 to 39, and just 56% of women aged 40 to 45. The decline reflects both fewer eggs and lower egg quality. This is why fertility specialists recommend seeking evaluation after 12 months of trying if you’re under 35, and after 6 months if you’re 35 or older.
Body Weight and Ovulation
Body composition directly affects whether you ovulate regularly. A BMI under 18.5 often causes irregular periods and can shut down ovulation entirely, because the body interprets low fat stores as a signal that conditions aren’t right for pregnancy. On the other end, obesity can also lead to irregular cycles and ovulation problems, largely through its effects on insulin and hormone balance.
The good news is that weight-related ovulation issues are often reversible. For underweight women, gaining even a modest amount of weight can restart regular cycles. For women with a BMI above 30, losing 5 to 10 percent of body weight frequently restores ovulation without any other intervention.
Nutrients That Support Egg Quality
Certain nutrients play specific roles in egg health. Folate is essential for DNA replication and helps prevent chromosomal errors during egg maturation. Vitamin D receptors are present in the ovaries and uterus, and low levels have been linked to lower fertility in multiple studies. Vitamin A supports the development of high-quality eggs, while vitamin E helps promote blood flow to the uterine lining.
CoQ10 has received particular attention because of its role in cellular energy production. Eggs are among the most energy-demanding cells in the body, and the energy-producing structures within them become less efficient with age. CoQ10 acts as both a fuel source for those structures and an antioxidant that protects egg DNA from damage. Animal studies show it can reduce cell death in the structures surrounding the egg, potentially improving both egg quantity and quality. Human research in IVF settings has shown promising results, though the benefits for natural conception are still being studied.
Putting It All Together
Fertility isn’t one thing. It’s the result of eggs, hormones, anatomy, timing, and overall health all functioning in sync. Some of these factors, like age and genetic egg supply, are outside your control. Others, like body weight, nutrition, and timing intercourse during the fertile window, are directly within it. Understanding which pieces of the puzzle matter most gives you a clearer picture of where to focus, whether you’re planning ahead or actively trying to conceive.