Diminished luteinizing hormone biopotency in breastfeeding women

The effect of nursing on plasma luteinizing hormone bioactivity (B-LH) and immunoactivity (I-LH) was assessed at 4 and 6 months post partum in fully nursing women who experienced their first bleeding between months 5 and 6 post partum (n = 6, short amenorrhoea) or after the month 6 (n = 10, long amenorrhoea). Controls were 10 non-nursing fertile women. Blood samples were drawn twice weekly at month 4 post partum and at month 6 post partum. In the nursing women who were cycling at month 6 and in non-nursing women samples were drawn during the follicular phase. I-LH was measured by a time resolved immunofluorometric assay (DELFIA) and B-LH by the mouse Leydig cell assay. Nursing decreased B-LH more than I-LH resulting in a relationship between B-LH and I-LH different to that of non-nursing women (B-LH = 2.84 x I-LH-0.16 and B-LH = 4.27 x I-LH + 3.11 respectively, P < 0.05, by likelihood test). Plasma B-LH or I-LH were similar in nursing women with short or long amenorrhoea. In conclusion, nursing alters the quality of circulating LH, however, the decreased LH steroidogenic potency does not play a role in determining the duration of lactational amenorrhoea.