Risk of pregnancy whilst breast feeding

Lactational amenorrhea results in a 2% pregnancy rate when used under 3 conditions (1):

• the lactating woman is supplying at least 90% of the infant's calories through breastfeeding at intervals no longer than every 4 hours during the day and every 6 hours at night, but ideally more frequently
• the lactating woman has not resumed her menses
• she is in the first 6 months postpartum
  
  
• such women may not need additional contraception, therefore avoiding the controversy of taking hormones while breastfeeding (1)
  
  
• pregnancy rate increases to 5% in working women even if they express their milk every 4 hours, suggesting that the suckling of the infant contributes substantially to the contraceptive effect
  
  
• beyond 6 months, the likelihood of ovulation preceding menses increases with time, raising the probability of conception
  
  
• the low pregnancy rate of lactational amenorrhea, in addition to the many other benefits of breastfeeding, may be another reason for women to consider strict breastfeeding (1)

Reference:

• 1) Pallone SR, Bergus GR.Fertility awareness-based methods: another option for family planning. J Am Board Fam Med. 2009 Mar-Apr;22(2):147-57.
• 2) Freundl G et al.State-of-the-art of non-hormonal methods of contraception: IV. Natural family planning. Eur J Contracept Reprod Health Care. 2010 Apr;15(2):113-23