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.
A normal menstrual cycle lasts 28 days, plus or minus seven days. Menstrual bleeding is considered irregular if it occurs more frequently than every 21 days or lasts longer than 8 days. Missed, early, or late periods are also considered signs of an irregular cycle. To determine whether your menstruation schedule is irregular, count from the last day of your previous period and stop counting on the first day of your next. Repeat this for three months.