Penelope Ody's book Home Herbal p. 140:
Senna: "stimulating, laxative. Caution: avoid in pregnancy, unless
prescribed by professional practitioner."

Prescribing it for constipation appears appropriate if patient is being
followed.  At several locations in her book she repeats the caution
indicating senna and rhubarb root laxatives should be avoided in
pregnancy and at one location she calls them "strong laxatives" and "if
pregnant, use butternut" instead (p.104).

I'd be inclined to do a thorough fluid intake, diet and exercise history
and educate patient on how to avoid constipation by drinking plenty of
fluids, increase exercise and eat a diet with foods in as close to their
natural state as possible (constipation is the curse of processed foods).

E-mail privately if you are interested in obtaining Home Herbal.

Bonnie Douglas, LLLL