The contraceptive effects of breast feeding are probably due to the marked changes which suckling causes in the pattern of luteinizing hormone (LH) secretion, studies conducted in Scotland have shown.  In mothers who fully breast feed their babies, these changes are sufficient to offer contraceptive protection of 98% for about 6 months postpartum. Breast feeding mothers should start to use another method of contraception 6 months after giving birth, or after they start supplementing breastfeeds with other food, or after the 1st menses, a plenary session on breast feeding, birth spacing, and child survival heard.  The researchers from Scotland also presented striking new evidence of the protective effect of breast feeding on the heaLth of newborn babies.  The studies were conducted in Edinburgh and Dundee over a period of more than 10 years and covered a range of different aspects of breast feeding and fertility.  Dr. Anna Glasier, who presented findings on the hormonal changes which occur in breast feeding women, showed that suckling postpones the return of ovarian activity after childbirth.  The results for 1 mother who bottle fed her baby showed that menstruation returned as early as 6 weeks postpartum, ovulation occurred at 10 weeks, and that by 14 weeks, a normal ovarian cycle had returned.  In a group of 27 breast feeding mothers, the onset of ovarian activity was found to coincide with the introduction of solid food and a decline in suckling activity.  In many cases, ovulation suppressed for several months after supplementation of breast feeding began and a normal ovarian cycle did not return until well after breast feeding had been stopped completely.  Dr. Glasier said all the women studied showed marked disturbances of the pulsatile pattern of LH secretion, which gradually returned to normal as suckling declined.  The disturbance is thought to be caused by the effect of suckling on the hypothalamic pulse generator, although the mechanism by which this occurs is not yet understood, she said.  The Scottish researchers believe that it is this disturbance which is responsible for the contraceptive effect of breast feeding.  Their findings appear to disprove the theory that ovarian suppression is due to the increased levels of prolactin which occur during breast feeding.  Dr. Glasier explained that the administration of exogenous GnRH restored normal LH secretion patterns and ovarian activity in women who were fully breast feeding their babies, despite the presence of elevated prolactin concentrations.   Her fellow-researcher Prof. Peter Howie presented the findings of a study of 750 Dundee mothers and their babies, aimed at examining the effects of breast feeding on the infants.  Because most previous studies have been criticized as methodologically flawed, the Dundee investigation was carefully designed to meet important methodological criteria.  Prof. Howie told the meeting that in the babies who were bottle fed or weaned before 13 weeks, gastrointestinal illness was 5-8 times more common than in babies who were breast fed for longer.  The rate of illness among the babies breast fed for less than 13 weeks the close to that in some developing countries.  The protective effect of breast feeding persisted beyond the period of breast feeding itself up to 1 year of life.  These data support the view that breast feeding of offers important health benefits to babies in all environments, she said.