A risk factor is something which increases the likelihood of developing a condition or disease. For example, obesity significantly raises the risk of developing diabetes type 2. Therefore, obesity is a risk factor for diabetes type 2.
Any woman who is pregnant has the potential for developing morning sickness. However, the risk is higher if:
Before the pregnancy the woman had experienced nausea or vomiting from: motion sickness, migraines, some tastes or smells, or birth control pills.
The woman had experienced morning sickness in a previous pregnancy.
The woman is expecting twins, triplets, quads, etc.