Ultimately its SIL's decision and she should be discussing her options in collaboration with both her Neurologist and her Obstertrician.

Obviously its not a life or death situation otherwise Babe would have been born by now. Its more of a "hinderance" situation due to the vomiting and dehydration.

I'm no Dr but its sounds to me like it probably is pregnancy related IF the condition wasn't pre-existing.

We all know that term is considered 37 weeks for 42 weeks but thats not to say that by delivering a baby at 37 weeks isnt without risks. Each baby is different. Although considered term at 37 weeks the babe can still be at risk for Respiratory Distress aswell as Small for Gestational Age as well as other issues that can come with having a pre-term baby.

The longer that babe stays in the better. Simple.

As for the delivery. I'm in 2 minds. Being a nuerological issue, going through the "stress" of labour might aggrivate the issue causing more stress for mum and therefore baby. However, if its believed that SIL's migraines can be controlled throughout labour and this is SIL's preference, then a vaginal birth would be an ideal option.

Again though...SIL needs to sit down with all involved to be able to come to a safe, educated decision.

All the best!
xx