Trouble Gaining Weight

Ember🐢 • Mama of twin boy and girl💙💜 Expecting baby #3 March 2019..💛
I'm 17+5 and I am having an awful time gaining weight. I eat plenty but I can't seem to do it. I've weighted the same since high school. And now I am just worried there will be something wrong with my babies because of my lack of weight gain. Can anyone help?!