If you use your imagination, getting your period is a lot like emptying out the lint trap inside of a clothes dryer, only the lint trap is your uterus and the lint is your uterine lining. Over time, the lint builds up and sticks to the walls of the trap, just like menstrual blood and tissue build up on the walls of your uterus every month.
If you don't clean your lint filter, then it's bad because lint is really effin' flammable. But sometimes, you may not get your period. In which case, what happens to your period blood? To seriously answer this, you've got to understand what causes you to miss a period in the first place.
Broadly speaking, a missed period happens because the hormones in a person's body are "not cycling normally," explains Kelly Treder, MD, MPH, instructor in the department of Ob/Gyn at Boston University/Boston Medical Center. There are lots of reasons why this happens. For example, any medical condition that messes with hormones — like polycystic ovarian syndrome or thyroid disorders — could change periods. Of course taking hormonal birth control can change your period, either making it lighter or heavier. Also, young people who are new to periods may have irregular cycles, and people approaching menopause skip periods altogether, too, she says. "One missed period might just be an 'off' month, but irregular periods should be evaluated by a gynecologist," she says.
If a person's hormones aren't cycling normally for whatever reason, then the lining of the uterus may grow, but the signals won't work to cause it to shed in the form of a period, says Rachel Cannon, MD, instructor in the department of Ob/Gyn at Boston University/Boston Medical Center. "In that instance the tissue stays in the uterus and causes the lining of the uterus to appear thickened," she says. Doctors can use an ultrasound machine to literally see that the lining of the uterus is thicker.
Eventually, that thick lining has got to go somewhere. "The lining of the uterus may be thickened and cause a heavy period with the next cycle," Dr. Treder says. You may notice that your period blood is a different color when this happens, and that's simply because it's been sitting around for a while.
On the other hand, not everyone experiences a heavy deluge or thicker period blood after a missed period. If you're using a hormonal birth control method, for example, you might not get a period because hormones are making your uterine lining super thin, Dr. Cannon says. "In other words the hormones do not allow the lining of the uterus to grow or build up each month," she says. You may find that your period blood is totally normal or even thinner after a missed period.
So, luckily it's not a big deal if you miss your period and notice that your period blood is thicker or darker the next time around. But it is a big deal if you don't clear out your lint trap, because that stuff can start a fire.