Copied from NHS website -
Lactational amenorrhoea method (LAM)
Women don't have periods while they're breastfeeding (this is known as lactational amenorrhoea), so breastfeeding can be used as a form of contraception. This is known as the lactational amenorrhoea method (LAM).
The fertility signals used in natural family planning methods are not reliable in women who are breastfeeding.
Women who are fully (or nearly fully) breastfeeding can use the lactational amenorrhoea method for the first six months after their baby is born, as long as:
the woman has complete amenorrhoea (no periods at all)
she's fully or very nearly fully breastfeeding (this means that the baby is having breastmilk only, and very little or no formula)
the baby is less than six months old
When used correctly and consistently, one in 200 women who use LAM will get pregnant in the first six months. However, take care to use the method correctly. Don't feed your baby other foods because this may reduce your lactation.
LAM becomes unreliable when:
other foods or liquids are substituted for breastmilk
your baby reaches six months old