My morning sickness was different with every child. With my oldest,I never had a bit of morning sickness. With the next one, I literally had to eat something every two hours throughout the entire pregnancy or I would be sick. With Cara, I started having morning sickness (and occasionally afternoon sickness)from week 7 or 8 and it subsided by week 15 or 16. This time, morning sickness was my first symptom. I'm only up to week 10, and I get nauseous almost every morning (don't always throw up, but I feel quesy). If I have a big lunch or dinner I usually throw up about half an hour later.
So basically, any way you decide to write it, it would be believable.
Hope this helps
AMR
