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.
Most women have between 11 and 13 menstrual periods each year. You may be different: You may have more or fewer. Missed or irregular periods must be looked at in terms of what is normal for you. Menstrual periods are often irregular during the first few years after menstruation starts.