In reality, there are probably hundreds of "causes" of infertility. What this means is that there are a lot of things that have to happen perfectly in order to conceive and have a baby.
Infertility is most commonly caused by problems with ovulation (the monthly release of an egg). Ovulation problems can occur as a result of a number of conditions.
For the first time, scientists have shown that it's possible for two people of the same sex to create a baby, without the need for outside egg or sperm donation. The most obvious benefits would be for homosexual couples who want to have a child together, but the method could also help couples who have been affected by infertility.