Does thc stay in a fetus system longer then the mother and if so how long?

Not sure: I'm not sure this question has been studied -- given that you'd have to expose a group of fetuses to a potentially damaging drug to find out. Main problem is that fetal brains are not well developed, and more vulnerable to toxins than the mother's brain is. The mom might feel thc is just fine for her, but her baby is not able to deal with it as well. I would imagine it remains at least 1 month.

Some basics: THC attaches to the fat cells in the user and leaks back out from the blood over time. A significant amount is cleared into the poop. If I am asked to look for any drug exposure in a newborn, I can usually find at least trace evidence for any significant maternal exposure after the first 12 wks. That is far beyond the mothers retention if she abstains late in pregnancy (up to ~1 mo).