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A girl can technically only get pregnant when she is ovulating. So it might seem like there is only a small window in which pregnancy can occur. But that's not entirely true.

Most girls have no idea when they are ovulating. Some girls ovulate at funny times. Some even ovulate during their period.

Sperm can also live in the v****a for almost a week.

And that means you should assume your partner is fertile and use protection every single time. Birth control is not hard to get, so there is no excuse for not using it.

I'm taking Human Sexuality at my college, and we just talked about this. Sperm can live inside the v****a for up to 6 days. Most women have a 28-32 day cycle and ovulation can take place between days 11-21. Generally it starts about 10 days before their period.

But everyone is a bit different, and there is the possibility of sperm staying alive inside the v****a. So even if she wasn't ovulating when sex occured, there is a chance the sperm could stick around until she is ovulating. It would take months of tracking to determine a somewhat-solid ovulation chart for someone.

You keep saying she goes for months, sometimes up to a year with no periods, right? And I have told you multiple times that women who go three or more months without a period need to see an ob-gyn. She could have a serious issue that is threatening her health, making it so that she can't get pregnant, or both.

Before trying to get pregnant:
-She should be taking prenatal vitamins
-She should be tracking her cycle as much as possible
-She should be seeing an ob-gyn
-You two should have the money and room for another person in your home
-You two should ideally have a stable relationship.
-You two should ask her ob-gyn questions, do some research on the internet, read some baby books, or all of the above.

It doesn't sound like you two have any of those things in place yet, so I have no idea why you are even trying. Trying before you are ready is selfish and not fair to the baby. But if you are going to keep trying without being at all prepared, without doing your research, without taking any of our advice, and without having her see a doctor, then at least stop complaining about the "mystery" behind why your on again off again partner isn't pregnant.

Women need to see an ob-gyn if they go three or more months without a period, if they are about to start trying for a baby, and for regular paps. So she needs to go in now, not just "if that continues to not work."