system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does smoking while being pregnant affect the baby?
gpt
Smoking reduces the amount of oxygen that is available to a growing fetus, which it does in two different ways.  First, the carbon monoxide in the cigarette smoke makes the blood less capable of carrying oxygen, and secondly, the nicotine acts to make blood vessels constrict and become narrower.  The reduction in oxygen makes it harder for the fetus to develop.