18 weeks and still can't feel the baby

Xiomara • I am 27 years old and my husband and I have two beautiful children. Our daughter is 8 and our son is 7. We are TTC baby #3.
I'm getting nervous because this is my 3rd child and they say if you had a baby before that you can feel the baby sooner but I haven't felt any movement. I was thinking of buying one of those fetal movement machines but this is our last child and I don't want to buy it and not need it in the future. What do you think?