Hi, I have understood your problem.
Are you on iron preparations? Usualy iron is prescribed during
pregnancy as it is essential. Iron is known to give blackish dark colour to the stool and many times causes pain in abdomen. Please do not be worried about dark stools. if you are too much bothered by the pain in abdomen, you may change the brand of the iron preparation. Time released iron spansules are available. These spansules have very low side effects. You may switch over to that.
In whichever form, iron should be taken about half an hour before meals - preferably dinner. Taken after food hampers the iron absorption. Also, while on iron, please do not take antacids as for better iron absorption, stomach must have acidic medium.
If your stools are dark without iron preparation, it indicates bleeding inter upper gastrointestinal tract. In that case you have to report to your Obstetrician and get investigated for the responsible cause which can be ulcer, hook worms, etc.
I hope you got the answer to your query.