Understanding Your Menstrual Cycle: Periods, Ovulation and Your Fertile Window

What is a normal cycle length, when does ovulation happen and how long is your fertile window? A clear guide to your menstrual cycle and when to get it checked.

Your menstrual cycle is more than your period. It is a monthly rhythm of hormones that affects energy, mood, skin, sleep and fertility. Understanding the basics can help you plan, notice changes early and have better conversations with your health care provider.

To estimate your next period, likely ovulation day and fertile window, try our free period and ovulation calculator. The guide below explains how those estimates are made.

What counts as a normal cycle?

A cycle is counted from the first day of one period to the day before the next one starts. In adults, a typical cycle lasts anywhere from about 21 to 35 days. A period itself usually lasts between two and seven days.

Only some people have a "textbook" 28-day cycle. Your cycle can vary by a few days from month to month and still be considered regular. Cycles are often less predictable in the first few years after periods start and again in the years leading up to menopause.

The phases of your cycle

• Menstrual phase: the lining of the uterus sheds, causing your period.
• Follicular phase: starts on day one of your period. Hormones prompt the ovaries to mature an egg, and the uterine lining thickens again. This phase varies the most in length.
• Ovulation: an egg is released from the ovary. The egg survives for about 12 to 24 hours.
• Luteal phase: after ovulation, the hormone progesterone rises. If pregnancy does not happen, hormone levels drop and your next period begins. This phase is usually more consistent, at roughly 14 days.

Because the luteal phase is fairly steady, the difference between a short and long cycle mostly comes from the follicular phase. That is why calculators estimate ovulation by counting back about 14 days from your expected next period, rather than forward from your last one.

Your fertile window

Your fertile window is the time each cycle when sex could lead to pregnancy. It is about six days long: the five days before ovulation and the day of ovulation itself. This is because sperm can survive in the reproductive tract for up to five days, while the egg lives for only about a day.

The days just before and including ovulation are usually the most fertile. If you are trying to conceive, having sex every two to three days throughout your cycle means you do not need to pinpoint ovulation exactly.

Apps and calculators are estimates

Period apps and calculators, including ours, work from averages and the dates you enter. They cannot detect ovulation directly. Their predictions are less accurate if your cycles vary, if you have recently stopped hormonal birth control, after pregnancy or while breastfeeding, or during perimenopause.

Some people add body signs to get a better picture, such as changes in cervical mucus (which becomes clear, stretchy and slippery near ovulation), basal body temperature (which rises slightly after ovulation) or urine ovulation tests. Even combined, these methods are not a reliable form of birth control unless you have been taught a specific method and use it carefully.

Irregular cycles and PCOS (PMOS)

Irregular periods can have many causes, including stress, big changes in weight or exercise, thyroid conditions, certain medications, breastfeeding and perimenopause.

One common cause is polycystic ovary syndrome (PCOS), which was recently renamed polyendocrine metabolic ovarian syndrome (PMOS) to better reflect its hormonal and metabolic effects. Signs can include infrequent or absent periods, excess facial or body hair, acne and difficulty with weight. PCOS/PMOS is also linked with a higher risk of type 2 diabetes, so getting a diagnosis matters beyond fertility. It is a treatable condition, and support is available.

Tracking that helps your appointment

Tracking for two or three months gives your care provider useful information. Note:

• the first day of each period and how many days it lasts
• how heavy the flow is (for example, how often you change pads, tampons or cups)
• bleeding or spotting between periods or after sex
• pain, mood changes, headaches or other symptoms, and when they happen in your cycle

When to get medical advice

Talk to your doctor, nurse practitioner or a sexual health clinic if:

• your cycles are regularly shorter than 21 days or longer than 35 days
• you have gone three months or more without a period and you are not pregnant
• your periods suddenly become irregular after being regular
• you soak through a pad or tampon every hour or two, pass large clots, or bleed longer than seven days
• you have bleeding between periods, after sex or after menopause
• period pain stops you from doing everyday activities
• you have been trying to conceive for 12 months (or six months if you are 35 or older)

Get urgent care if you have very heavy bleeding with dizziness or fainting, or severe pain in your lower abdomen, especially if you could be pregnant. In an emergency, call 9-1-1.