Iregular periods are much more common that you think. The pill regulated your hormones causing menstruation and ovulation and therefore your cycle on birth control pills are much more regular than without it.

There are various resources available that will teach you how to recognize ovulation. You can only get pregnant a few days before and after ovulation. It is thus important that you have sex around your ovulation date. For this you will have to keep a chart of your temperature, mucus secretions and the position of your uterus or some combination of these.

Also remember to have sex for fun and not only to make a baby! Just concentrating on making a baby can become pretty boring and all consuming after a while.

You can also visit your health care provider that can determine if you ovulate and if your partner's sperm is healthy.