This is more of a misunderstanding than a myth.
The hormone prolactin, which stimulates lactation, also squashes the release of a chemical necessary for ovulation (GnRH). But -- and this is a big but -- a woman must meet three criteria for breast-feeding to work as a form of contraception, known as the lactational amenorrhea method (LAM) [source: Association of Reproductive Health Professionals]:
- She hasn't begun to menstruate again.
- The baby is younger than 6 months old.
- She is engaging in continuous breast-feeding.
Continuous breast-feeding means feeding the baby exclusively from the breast at least every four hours during the day and every six hours at night. No pumping.
If even one of these criteria isn't met, it's possible to get pregnant while breast-feeding, so if the idea of another baby so soon isn't appealing, break out the birth control.