Dear mothers, who took vitamins in the third trimester?

Dear mothers, who took vitamins in the third trimester? Was it prescribed by a doctor or did you take them yourself? Do vitamins affect the baby's weight at birth? With our first son, we didn't take any vitamins during pregnancy. He was born at 3120 grams and 50 cm. Now he has a very small belly too, will vitamins help him gain weight? We are 30 weeks pregnant.