How fertility actually works.

What fertility means

Fertility is the ability to conceive a child. For women it depends on releasing a healthy egg. For men it depends on producing sperm that can reach and fertilize it. Pregnancy also needs open fallopian tubes, where egg and sperm meet, and a uterus where an embryo can implant and grow.

When pregnancy does not happen, the reason can lie with either partner, with both, or remain unexplained.

One cycle, step by step

A menstrual cycle is the body preparing for a possible pregnancy, once a month.

• Your period starts. The uterus sheds last cycle’s lining, and a group of follicles in the ovaries begins to grow.
• One follicle takes the lead. As it grows it produces estrogen, which thickens the lining of the uterus.
• A hormone surge triggers ovulation: the follicle releases its egg into the fallopian tube.
• If sperm are present, the egg can be fertilized in the tube. The embryo travels to the uterus and implants about a week later.
• If there is no pregnancy, hormone levels fall, the lining is shed, and a new cycle begins.

The hormones in charge

Four hormones do most of the work. Follicle-stimulating hormone (FSH) tells the ovaries to grow follicles. Luteinizing hormone (LH) triggers ovulation. Estrogen builds the uterine lining, and progesterone, produced after ovulation, keeps it ready for an embryo. Fertility tests measure some of these, which is why they are timed to particular days of your cycle.

What affects your chances

Age matters most. According to the American Society for Reproductive Medicine, a healthy, fertile 30-year-old woman has about a 20% chance of getting pregnant in any single cycle. By 40 the chance is less than 5% per cycle. The reason is that both the number of eggs and the share of them that are chromosomally normal decline.

Other things count too: conditions such as endometriosis and polycystic ovary syndrome, blocked tubes, thyroid problems, smoking, very low or very high body weight, and sperm quality.

What you can do with this

You cannot change your age, but you can know where you stand. A fertility assessment replaces averages with your own numbers, and options such as egg freezing exist for women who want to wait. Neither is a guarantee. Both are ways of deciding with information instead of worry.