Query: Are you dieting or under any other form of stress?
In reverse order from least to most important
2) Stress nixes most women's periods.
Typically, docs don't worry about irregular periods for the first 3 years after birth (after checking hormone levels and pregnancy). Why? Because being a new parent with sleep dep throws most women's periods out of whack even if they never breastfeed. The first 2 years are a neurological mess, and then 1 year to get back in the rhythm of things, then worry after that.
1) No period from dieting, however, is quite dangerous. Not in and of itself... but It's a sign that you're 'eating' your own organs by the time you lose your period. Once you've lost your period, you've already started getting heart and muscle damage, and organ damage.
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There are tons of other causes for irregular periods, most fairly harmless. The thing to keep an eye on, however, is whether or not you're pregnant. I was 4 months pregnant at one point and didn't know it because my period was irregular. I didn't get sick, sore, fluttery, emotional... it was a very "easy" pregnancy until month 6. A lot of moms jaws drop when they hear about people not knowing... but when you have a super easy 1st trimester, and are used to irregular periods... women are often 16-20 weeks before they find out. Some for even 7-8 months if the uterus grows upward instead of outward. We don't have the NERVES in our abdominal cavity that we do in our pelvic area. You DON'T FEEL KICKS. Or flutters. Or shifts. There's no pressure on your bladder, so you're not peeing all the time. There's no pressure on the nerves or bloodflow return for your legs, so you don't have swollen feet/legs... and don't have most of the aches associalted with having a beach ball hanging off your front. Most women can still wear their 'prepregnancy' clothes without them even being tight at all until the very end, and even then, it's only about a half size difference. Really, it would be better all around to carry babies this way... the problem is that births tend to be very difficult... but a baby growing "up" instead of "out" is more common than having a baby with downs. Some eventually shift "out", many don't.
The point here, is pretty obvious. If you have no symptoms of pregnancy (missed periods, morning sickness, discomfort, etc.)... will you take care of yourself in the same way? Avoid alcohol, cold meds, rx meds, etc.? Most people don't LIVE like they're pregnant. So if you don't have regular periods, it's recommended that you test for pregnancy every 2-4 weeks.