Is It Safe To Take Clexane Injections In Pregnancy?

Hi, I'm 26 weeks pregnant and I have high blood pressure my baby isnt growing well and Im taking Aldomet tablets and my doctor prescribed me to take clexane injections but I've been reading about them and it saying all over the place to not take it once pregnant. Why did my doctor prescribe me clexane im sure he told me but he told me so much that i forgot.