Oral Progestogen-only Contraception May Protect Against Loss of Bone Mass in Breast-feeding Women

Background And Objectives: A worldwide trend towards increasing life expectancy has meant that osteoporosis is emerging as an important public health problem. The loss of bone mineral density and its restoration in association with a premenopausal but physiological hypo-oestrogenic state may serve as an important model for research into the pathogenesis and prevention of osteoporosis. With this in mind we have undertaken a longitudinal study of changes in bone mineral density over one year in women after childbirth.

Design: Observational study of 31 women in the first year following childbirth; 11 intending to breast-feed and use barrier methods of contraception, 9 intending to breastfeed and to use the progestogen-only pill and 10 intending to artificially feed and to use barrier methods.

Patients: Recruitment was from the antenatal clinics of the Simpson Memorial Maternity Pavilion. Only non-smokers who had regular menstrual cycles prior to conception were included.

Measurements: Bone mineral density was measured at the lumbar spine within 3 weeks of childbirth and repeated at 6 and 12 months post partum. Plasma oestradiol, prolactin and osteocalcin concentrations were measured at each visit.

Results: Breast-feeding women using barrier methods lost a mean +/- SE of 4.9 +/- 1.5% of bone mineral density in the first 6 months following delivery. This was however reversible since by one year the bone mineral density was no different from that measured immediately post partum. Breast-feeding women using the progestogen-only pill lost a significantly smaller percentage of bone mineral density in 6 months and by one year bone mineral density was 2.95 +/- 0.75% higher than post partum. Artificially feeding women had a steady increase in bone mineral density in the first year and bone mineral density was on average 4.3 +/- 1.2% higher.

Conclusion: Breast-feeding results in a reversible reduction in spinal bone mineral density. The small amounts of gestagen in the progesterone-only pill would appear to protect against this loss. The mechanism of this loss in bone mineral density and the potentially bone protective effects of gestagens require further study.