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.
Irregular periods can be a signal that something isn't quite right with your reproductive system. In fact, for some women, irregular periods are an early infertility symptom. That said, there is also a wide range of normal. Understanding what's considered irregular and what's not can help you understand your body.