To become pregnant, an egg must be fertilized by a sperm. This can happen in different ways.
To become pregnant you should have unprotected intercourse or inseminate sperm days before ovulation. For those who regularly menstuation ovulation ocurs approximately 14 days before the next expected menstruation.
In case of irregular ovulation one may take help of, for example, an ovulation test to find out when next ovulation will take place and when the possibility of getting pregnant is greatest.