Using breastfeeding as a means of contraception is known as the lactational amenorrhea method (LAM). When you breastfeed, your body releases a hormone that suppresses oestrogen production, thereby interfering with your menstruation cycle. The effectiveness of this ovulation suppression is very much related to how much you nurse your baby. As soon as you start supplementing your baby's diet with solid food, or feeding formula, your body senses the change in milk demand and compensates for this. That means less milk gets produced, and less suppressing hormones released. It is therefore a good idea to use other methods of contraception in addition to LAM after the first couple of months.
LAM is fairly effective for the first six months, with less than two percent of breastfeeding mothers becoming pregnant during the first six months following delivery. Barrier methods and the progestin-only pill are completely compatible with breastfeeding.
To make LAM as reliable as you can, the following conditions must be met: There is no more than a four hour gap between feeding; your period has not started again; your baby was delivered less than six months ago. Remember that your body can release an egg without there being signs of menstruation returning, thereby making you potentially fertile.
Do not use LAM if you express your milk, or use formula feeding as well as breastfeeding. LAM depends on long, regular feeding sessions, so substituting even one feed with a bottle can be risky.