The resumption of post-partum menstruation and ovulation was studied in ten bottle feeding and twenty-seven breast feeding mothers. First menstruation occurred at 8±1 weeks (± 1±0 SE) in bottle feeders and 32±5 weeks (±2±5 SE) in breast feeders (P < 0±001); first ovulation occurred at 10±8 weeks (±1±0 SE) in bottle feeders and 36±4 weeks (±2±5 SE) in breast feeders (P < 0±001).
In bottle feeders, ovulation preceded first menses in only 2/10 (20%) of mothers but was regularly established thereafter, occurring in 17/18 (94%) of second and subsequent cycles.
Breast feeding did not postpone ovulation indefinitely because 13/27 of the breast feeding mothers ovulated while still lactating; ovulation occurred in 9/27 (33%) of breast feeding mothers during the phase of lactational amenorrhoea but was followed by menstruation in every case. In breast feeding mothers, the frequency of ovular cycles progressively increased with time, ovulation being observed in 9/20 (45%) of first cycles during lactation, 20/30 (66%) of later cycles during lactation, 16/23 (70%) of first cycles after lactation and 26/31 (84%) of later cycles after lactation.
There was a disruption of menstrual rhythms during lactation, the mean interval between the first day of consecutive menstrual cycles being 37±0 days ± 3±3 SE during lactation compared with 29±8 days ± 1±0 SE after lactation and 29±5 days ± 1±0 SE in the bottle feeding mothers.
This study shows that bottle feeding is associated with an early resumption of post-partum menstruation and ovulation. In breast feeding mothers, there is complete suppression of ovulation during the greater part of lactational amenorrhoea but ovulation will return in a proportion of mothers just before first menses. After the return of menstruation during lactation, the frequency of ovular cycles progressively increases but does not return to normal until complete weaning has taken place.