Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere. It is primarily composed of nitrogen (78%) and oxygen (21%) with only small concentrations of other trace gases.
Having irregular periods is very common. I see them in my practice all the time. The causes can range from something insignificant to something that requires treatment. For example, if you're a teen, your body's hormones can fluctuate and take some time until they find a balance.
If you've been having periods for 2 years or less and your physical exam is normal, your irregular periods may be part of your normal development. Remember, if you have had sex, go to your doctor's office for a pregnancy test when your period is late for you, even if you normally have irregular cycles.enstrual flow lasts about 3 to 7 days. A menstrual period is considered late if it is 5 or more days overdue according to your usual pattern of periods. A period is considered missed if there is no menstrual flow for 6 or more weeks.