Every body is different, I know that senna does NOT cause cramps in my body, as I have taken it for years.
Bisacodyl on the other hand causes major cramps for me and I wouldn't touch that in pregnancy with a barge pole!!
Physsillum husk and lactulose is no good cause my MS means I can't keep either down, a tablet has more luck, although it took three separate attempts to keep one down, then did they job without incident and I felt 100% better.
There are so many things during pregnancy that can be linked with miscarriage, but you just have to do what you know is right for your own body xxx