Being fertile means your body is capable of becoming pregnant. More specifically, it refers to the ability to release a mature egg, have that egg fertilized by sperm, and sustain a pregnancy. Fertility isn’t a fixed state, though. It shifts throughout your life, fluctuates within each menstrual cycle, and responds to everything from your age to your weight to underlying health conditions.
Fertility Across Your Lifetime
Girls typically reach puberty between ages 8 and 13, which is when menstrual cycles begin and the body becomes capable of reproduction. From that point forward, regular periods are generally a sign that the reproductive system is functioning normally. Fertility peaks in the early to mid-20s and then begins a gradual decline, with a sharper drop after 35.
The numbers tell the story clearly. In a large North American study tracking couples actively trying to conceive, about 71% of women aged 21 to 24 became pregnant within 12 cycles. That figure held fairly steady through the early 30s, with roughly 78% of women aged 28 to 30 conceiving in the same timeframe. By ages 34 to 36, the 12-cycle rate was about 75%. The real shift came after 40: only about 56% of women aged 40 to 45 conceived within a year of trying. That’s a meaningful drop, driven largely by a decline in both the number and quality of remaining eggs.
This gradual decline eventually ends with menopause, when menstrual cycles stop permanently, typically in your late 40s or early 50s. But fertility decreases well before that final period.
The Fertile Window Each Month
Within each menstrual cycle, there are only about six days when sex can result in pregnancy. This “fertile window” includes the five days before ovulation and the day of ovulation itself. The reason it stretches beyond ovulation day is that sperm can survive inside the reproductive tract for up to five days, waiting for an egg to be released.
The egg, by contrast, is far less patient. Once released from the ovary, it survives for roughly 12 to 24 hours. If sperm aren’t already in position or don’t arrive quickly, fertilization won’t happen that cycle.
A common misconception is that ovulation always happens on day 14 of a 28-day cycle. In reality, only about 30% of women have their fertile window fall entirely within the textbook range of days 10 to 17. On any given day between cycle days 6 and 21, a woman has at least a 10% chance of being in her fertile window. Cycle length varies from person to person and even month to month, which makes pinpointing ovulation a moving target.
How Your Body Signals Fertility
Your body gives off physical cues as ovulation approaches. The most reliable one you can observe at home is changes in cervical mucus. In the days before ovulation, discharge becomes wet, stretchy, and slippery, often compared to raw egg whites. This texture isn’t just a signal; it actually helps sperm travel more easily through the uterus toward the egg. When you notice that consistency, you’re at your most fertile point in the cycle.
Basal body temperature (your temperature first thing in the morning, before getting out of bed) also shifts around ovulation, but it rises after the egg has already been released, typically by less than half a degree Fahrenheit. That makes it useful for confirming that ovulation occurred, but not for predicting it in advance.
Home ovulation predictor kits detect a hormone surge that happens roughly 36 hours before ovulation. These kits are highly accurate. A study comparing five popular brands found accuracy rates between 92% and 97% when measured against blood tests. They’re a practical option if you want a clearer heads-up that ovulation is approaching.
What Can Reduce Fertility
Several medical conditions directly affect a woman’s ability to conceive. Polycystic ovary syndrome (PCOS) disrupts ovulation, sometimes causing irregular or absent periods. Endometriosis, where tissue similar to the uterine lining grows outside the uterus, can cause pain and interfere with fertility. Uterine fibroids, which are noncancerous growths in or around the uterus, can also contribute to difficulty conceiving depending on their size and location.
Beyond medical conditions, everyday lifestyle factors play a measurable role. Body weight is one of the most significant. A BMI under 18.5 (underweight) or over 30 (obese) can disrupt hormone balance and interfere with regular ovulation. Smoking, heavy alcohol use, and high caffeine intake have all been linked to reduced fertility. Chronic psychological stress can also throw off ovulation timing, though the effect varies from person to person.
Nutrition matters too. Research has found that women who eat more vegetable-based protein, healthy fats (particularly monounsaturated fats over trans fats), and iron-rich foods, while reducing processed carbohydrates, have lower rates of ovulation-related infertility. These aren’t miracle fixes, but they represent factors within your control.
How Fertility Is Measured Medically
If you’ve been trying to conceive for 12 months without success (or 6 months if you’re 35 or older), that meets the clinical definition of infertility. At that point, a doctor may check your ovarian reserve, which is essentially an estimate of how many eggs you have remaining.
One common test measures a hormone called AMH (anti-Müllerian hormone) through a simple blood draw. Average levels fall between 1.0 and 3.0 ng/mL. Levels below 1.0 are considered low, and below 0.4 is severely low. A low result doesn’t mean pregnancy is impossible, but it suggests fewer eggs are available, which can affect your options and timeline. AMH gives a snapshot of egg quantity, not quality, so it’s just one piece of the picture.
Other assessments might include ultrasound imaging to count developing follicles in the ovaries, or tests to confirm that the fallopian tubes are open and the uterus looks normal. These help identify whether a specific, treatable issue is behind the difficulty conceiving.