Fertility in women is the biological ability to conceive a child. It depends on a chain of events that must happen in sequence each month: an egg must mature and release from the ovary, travel into the fallopian tube, encounter sperm, become fertilized, and then implant in the uterine lining. A woman’s fertility is highest in her 20s and early 30s, then declines more sharply after 35 as both egg quantity and quality decrease.
How the Monthly Cycle Creates a Fertile Window
Fertility revolves around ovulation, the moment when a mature egg releases from the ovary. This typically happens about 14 days before the start of your next period. But fertility isn’t limited to that single day. Your fertile window spans roughly seven days total: the five days before ovulation, the day of ovulation itself, and the day after. Sperm can survive inside the reproductive tract for up to five days, which is why intercourse before ovulation can still lead to pregnancy.
The cycle leading up to ovulation is more complex than most people realize. Each month, about 1,000 immature follicles begin developing in the ovaries. These follicles compete with each other, and only one “dominant” follicle wins out. The rest break down and are reabsorbed. That dominant follicle grows, fills with fluid, and eventually ruptures to release its egg into the fallopian tube. If the egg isn’t fertilized within about 24 hours, it dissolves, and the body prepares to shed the uterine lining as a period.
The Hormones That Drive Fertility
Four hormones orchestrate the process. Follicle-stimulating hormone (FSH) kicks things off by telling the ovaries to develop follicles and prepare an egg. As follicles grow, they produce estrogen, which thickens the uterine lining and, when it rises high enough, triggers a surge of luteinizing hormone (LH). That LH surge is the direct signal for the dominant follicle to rupture and release the egg.
After ovulation, the empty follicle transforms into a temporary gland called the corpus luteum, which pumps out progesterone. Progesterone stabilizes the uterine lining so a fertilized egg can implant. If no pregnancy occurs, the corpus luteum breaks down after about two weeks, progesterone drops, and a new period begins. The drop in progesterone allows FSH to rise again, and the whole cycle restarts.
Egg Supply and Why It Matters
Unlike sperm, which men produce continuously, women are born with a finite number of eggs. The supply actually peaks before birth, at around 5 to 7 million during fetal development. By the time a baby girl is born, that number has already dropped to roughly 500,000 to 1 million. By puberty, about 400,000 remain. Of those, only around 400 to 500 will ever fully mature and ovulate during a woman’s reproductive years. The rest gradually break down over time.
This decline in egg count, called ovarian reserve, is one of the main reasons fertility drops with age. But it’s not just about numbers. Egg quality also decreases, meaning older eggs are more likely to have chromosomal abnormalities that can prevent implantation or lead to miscarriage. This is why women over 35 are generally advised to seek a fertility evaluation after six months of trying to conceive, while women under 35 are typically told to try for 12 months first. For women over 40, earlier evaluation is often recommended.
Signs Your Body Gives You
Your body produces visible clues about where you are in your cycle, and cervical mucus is the most reliable one to track at home. In the days after your period, mucus tends to be dry or thick and paste-like. As you approach ovulation, it becomes creamy and smoother. Right before and during ovulation, it turns clear, slippery, and stretchy, often compared to raw egg whites. This texture isn’t just a signal; it actually helps sperm travel more efficiently through the reproductive tract. After ovulation, the mucus dries up again.
Tracking these changes over a few cycles can help you identify your personal fertile window. Other signs include a slight rise in basal body temperature after ovulation (which confirms it already happened) and mild pelvic discomfort on one side during egg release.
Common Conditions That Affect Fertility
Several reproductive conditions can interfere with ovulation, implantation, or both. Polycystic ovary syndrome (PCOS) affects 4 to 7% of women of reproductive age and is one of the most common causes of irregular or absent ovulation. It involves hormonal imbalances, particularly higher-than-normal levels of androgens, that can prevent eggs from maturing properly.
Endometriosis affects about 10% of reproductive-age women and up to 50% of women experiencing infertility. In this condition, tissue similar to the uterine lining grows outside the uterus, causing inflammation that can damage the fallopian tubes, ovaries, or surrounding tissue. Uterine fibroids, which are benign growths in or on the uterus, are even more common, affecting 30 to 70% of reproductive-age women, though not all fibroids interfere with fertility. Their impact depends largely on size and location.
In 10 to 20% of infertility cases, no identifiable cause is found, a frustrating diagnosis known as unexplained infertility. These cases often share underlying inflammatory or hormonal disruptions that current testing may not fully detect.
How Fertility Is Tested
When a doctor evaluates fertility, one of the key blood tests measures anti-Müllerian hormone (AMH), a protein produced by developing follicles in the ovaries. AMH provides a snapshot of your remaining egg supply. The general ranges used to interpret results categorize levels at or below 1.1 ng/mL as low, 1.2 to 3.4 ng/mL as normal, and 3.5 ng/mL or above as high.
To put those numbers in context by age: the median AMH level for women aged 20 to 24 is about 4.2 ng/mL, dropping to roughly 3.5 ng/mL in the late 20s, 2.4 ng/mL in the early 30s, 1.3 ng/mL in the late 30s, and 0.5 ng/mL by ages 40 to 44. A low AMH doesn’t mean pregnancy is impossible, but it does suggest fewer eggs are available and may influence the urgency and type of treatment considered. Doctors also typically check FSH levels, thyroid function, and the physical condition of the uterus and fallopian tubes.
Lifestyle Factors With Real Impact
Body weight plays a measurable role in fertility. Women with a BMI over 30 have a significantly lower probability of live birth following fertility treatments compared to women in the 18.5 to 24.9 range. Excess body fat can disrupt hormone signaling and interfere with regular ovulation. Being significantly underweight can also suppress ovulation by reducing estrogen production. Many fertility clinics require a BMI below 30 before beginning treatment.
Smoking is one of the most damaging lifestyle factors for fertility. It is a major risk factor for infertility on its own, and during pregnancy, the risk of miscarriage increases by an estimated 1% for every cigarette smoked per day. Even secondhand smoke exposure has been linked to a substantially higher miscarriage risk. Alcohol consumption, diet quality, and physical activity levels also influence outcomes, though the evidence is strongest and most consistent for weight and smoking.