I had severe constipated and piles (thrombosed ones twice, lovely!) and DS came out a healthy 9 pounds. I also had an mmc at 14 weeks - no toilet problems whatsoever. So in my experienceis that constipation does not equal miscarriage.
piles/constipation tips:
use a footstool when you go to the loo, it raises your knees so you are in a more natural squatting position, you dont have to push so hard (never push!) and empties your bowels more fully.
eat lots of bananas, onions, garlic, dandelion leaves, leeks and artichokes- your intestinal flora feeds off these foods and flourishes. More intestinal flora means food is better broken down and poo is softer.
do eat lots of natural fibre like fruit and veg but NEVER eat processed fibre like Ryvita, All bran etc. This doesnt get broken down, swells your poo making them hard and large and can cause cuts and tears in your intestine and bowel (fissures) see. Processed fibre also strips away the intestinal flora, so you become dependant on eating more of it in order be able to go to the toilet.
aloe vera gel applied directly to your bum provides temporary cooling relief from piles - its not a cure but does ease pain. You can find it in the suntan section at the supermarket.