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A friend of mine asked me this today, and I didn't know. I remember studying the menstrual cycle a few years ago, but I can't remember the details (other than it was very hormone-related), and so I don't have a good base in the matter anyhow.

What makes some women have irregular periods? Why?

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    $\begingroup$ You get things like nutritional factors and cortisol (stress) intermingling with sex hormones. Also, sex hormone secretion is primarily steered by the hypothalamus (through pituitary) which may be influenced by the general state of your mind. Would be interesting to see if a woman actually has a regular cycle if she manages to maintain the exact same mood and diet. $\endgroup$
    – Armatus
    May 29, 2012 at 22:49
  • $\begingroup$ @Armatus true, that'd be pretty interesting, given that I read somewhere that 30% of women are irregular. You have to wonder if its always the same 30%. $\endgroup$
    – LanceLafontaine
    May 30, 2012 at 14:28
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    $\begingroup$ Actually, I'm pretty sure the rest of the brain plays a role there; be it through the hypothalamus or not. I've read about studies with students sleeping in the same room whose menstrual cycles apparently adapted to each other slowly until they all roughly menstruated at the same time. $\endgroup$
    – Armatus
    May 30, 2012 at 21:35
  • $\begingroup$ That's actually should be another question altogether, I'd be curious to know exactly what's going on there (if it's known). $\endgroup$
    – LanceLafontaine
    May 31, 2012 at 14:22
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    $\begingroup$ @Armatus: what you are describing is called the McClintock effect and the extent to which it actually exists are still under debate. A similar effect is a quite well known phenomenon in mice, and it is due to pheromones in urine. $\endgroup$
    – nico
    Jun 28, 2012 at 17:11

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There's two phases of a menstrual cycle, before ovulation known as the follicular phase and after ovulation known as the luteal phase. The second phase is not very variable, it's the same length almost always as it is governed by how long the corpus luteum (remnants of the follicles after the ovum bursts out) survives. The first part is variable and is what causes periods to be irregular. In this phase the eggs mature. The body picks several eggs to mature but only one will "win". However, how fast it wins changes. It all depends on how fast it grows and much it manages to suppress growth of the other eggs and that in turn depends on the quality of the egg. But then this also branches out, as it is controlled by levels of hormones this is heavily affected by stress and diet among other things.

It's unfortunate that the first phase is variable otherwise knowing which day you were going to ovulate would be easy.

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