Answer
No, it is an observational study.
We can say that: "Smoking during the third trimester is associetad with lower-birth-weight babies".
Work Step by Step
In order to claim causation we need to conduct a designed experiment. In this case, the researchers should control the smoking behaviour of the women: a selected group of women should smoke during the third trimester of pregnancy, while another group don't. But, this would be an unethical study.
