What are the side effects of taking iron tablets by pregnant women and what are the reasons for side effects?

futures currency trading said:

My wife is pregnant and is in her third trimester ( 7th month). Whenever she starts taking iron tablets, she starts feeling uncomfortable after consuming them for a week. Some times, she feels pain in stomach. Sometimes, she will have frequent nature calls. But, one doctor says that iron is very important and she should not stop them taking despite the side effects . Another doctor says that she should stop taking iron tablets and she will perform iron injections whenever needed. We are surprised that how can any one bear iron injections when they can’t be comfortable with iron tablets? We are in dilemma about following the advice of any of these two doctors.