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.
It's normal to have an irregular cycle or two now and then. But if you continue to have irregular periods, or if you don't have a period at all for three to six months, it's time to see your doctor. Most menstrual cycle intervals occur about every four weeks, with the normal range between 21 and 35 days. To calculate your menstrual cycle, start counting from the first day of your last period and stop counting on the first day of your next period.