Girlology: A Girl's Guide to Stuff that Matters

Cycle days 15–18. Ovulation has usually happened by now, and the egg is still traveling down the tube. If it has already hooked up with a sperm, an embryo is forming (we’ll talk about that more in chapters 6 and 7). Most likely, it’s alone and enjoying the massage as it is swished down the fallopian tube into the uterus. Once the egg arrives in the uterus, it hangs out for a couple days before it’s time to go.

Cycle days 18–28 or so. This is a time for rest in the endometrium. The follicle that had released the egg is now making a new hormone called progesterone. Progesterone will make the fluid in the cervix thicken and will get the lining in tip-top shape in case an embryo arrives. If there is no pregnancy in the uterus, the amount of progesterone falls and the endometrium can’t live without it, so it leaves the uterus as a period, which brings you back to day 1. As the progesterone levels are changing, some girls get moody or might feel more hungry than usual. Acne can worsen during this time. Some girls even feel a little puffy or bloated. That’s why you might hear girls complaining when they’re about to get their periods. All these symptoms are common and go away as soon as a period starts. Rarely, these symptoms are so bad that they affect your friendships and relationships at home. If that happens with you, please talk with your doctor because there are things that can help.

What if My Periods Are Not Regular?

Let’s back up a little. After your very first period (it’s called menarche, which sounds like anarchy and is pronounced MEN ar kee), you might have another period a month later, or it might not come again for several months.

After your first few periods, you should expect your periods to occur anywhere from 21 to 45 days apart. It might be 28 days one month, 40 days the next, 32 days the next. That’s all normal at first. A period doesn’t always come at the same exact time every month (remember months aren’t all the same length, either), but it should start to show a predictable pattern within a few years after you start.

If fact, the age you are at menarche can predict how soon your periods will start happening in a regular pattern.

Age at MenarcheYears until Very Predictable PeriodsUnder 111 to 2 years11 to 12Up to 2 or 3 yearsOver 12Up to 4 or 5 years

Periods will become “very” predictable once you start ovulating regularly. It’s not as reliable as the TV schedule or even as the trash pickup days, but it’s as reliable as they can get—that means within two to three days of the predicted time. Remember, to ovulate means to release an egg from your ovary. Once you start to ovulate with every cycle, then your periods become predictable. Your period will always start about 14 to 16 days after you ovulate. Problem is, you don’t always know when to expect ovulation. Like we said before, most girls have no clue when they ovulate because they usually don’t feel it. But if your periods are happening about the same number of days apart every time, you can know that you are probably ovulating. Miraculous!

So far we’ve been talking about regular ovulation, which means you are ovulating pretty much every month. But did you know you can ovulate before you even get your first period? After that, some of your cycles will include ovulation and some won’t. That means pregnancy can happen before a girl even has her first period if she is having sex.

Menstrual Calendar

Keep up with your periods by using the boxes below whenever you have bleeding. Copy it or make a chart like it to keep track of your bleeding—the heavier your bleeding is, the darker you fill in the box. Very light days (some people call this “spotting”) can be shown with just a dot in the box for that date. If you develop any irregular bleeding, this chart or one like it will be very helpful to your doctor. Besides, it will also help you recognize the pattern of your periods and help you predict when you will have your next one.

Predicting Your Periods

By keeping track of your periods on a calendar, you can usually tell when you should expect your next period. To calculate your cycle length, look at the past few periods on your calendar. Starting with the first day of a period, count the number of days until the first day of the next period. Do this for two or three cycles.

Most cycles will be between 21 and 45 days long. If your cycle length is about the same (within three to five days of each other) for several cycles, you can then start predicting your next periods pretty accurately.