Measuring small

Has anyone dealt with severe anemia which resulted into your little one measuring behind? My blood work came back as low as 9.6 and between 7-8 is when they do a transfusion. My son is measuring about 2 weeks behind and there worried about him having a low birth weight which right now he is at 3 lbs 8 ounces as I am 31 weeks along. I have started taking Iron but I feel so guilty that I didn’t get my blood work done weeks ago like I should have. The doctor today mentioned that a low birth weight heightens the chance of stillborn and I’m mentally freaking out. This is suppose to be our last baby, his heart rate was good and he’s still very active. Please some words of encouragement.