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.
About Irregular Periods. When discussing irregular periods, it is helpful to first understand what a normal period is. While every woman is different, normal periods are typically described as occurring every 28 - 31 days, with bleeding lasting approximately five days.