Hi,
If you are fully breastfeeding and your baby is having no formula or food and your period has not returned it is VERY unlikely that you would become pregnant until your baby was over 6 mnths of age. It would be extremely unlikely that you will be fertile in the first 4-6 weeks post birth.
Under 6 months, if if you do have a period it is likely that you will not have ovulated before you have your period. (this changes after 6 mnths.) The problem comes when mothers start a bottle, and therefore there is a longer gap between feeds, or when they wean but don't realise they will need extra contraception without the protection of breastfeeding