My single daughter is about to become a divemaster and is not pregnant
thank goodness.  I will ask the experts at the College she is attending for
a 13 month course. However I doubt they will know.  I know my daughter who
has carried out 60 dives is very fit and well.

More academically there is an article or two on exercise and breastfeeding
and the findings suggest that the supply improves with exercise.

Lovelady, C.A Lonnerdale. and Dewey  1990  Lactation performance of
exercising women.  American Journal of Clinical Nutrition  Vol 52 103-9

and
Dewey, Lovelady et al l994  A randomised study of the effects of aerobic
exercise by lactating women on breast-milk volume and composition  N Eng J.
Med 2:17:330 (7) 449-533