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.
So if your periods are consistently irregular, it's worth talking with your doctor â they may be able to test you for one of the above health issues or a different one, and keep you from having to go through a monthly freak out every time your period is running a smidge late.