I am very sorry for your loss. This was very tragic. It is difficult to comment on the exact cause of death at this point. It could be head injury or some other internal injury.
You are being very brave by thinking ahead. Planning another child will help you recover from this loss.
For planning to conceive, you need to control the TSH. Please see an endocrinologist
or let me know her TSH levels and the current dose of medications she is on. If she has regular menstrual cycles, start trying naturally (the best time to do so is the middle 10 days of the cycle). It is a good idea to get some basic tests done if you don't conceive in 3-4 months.
Please let me know if you need any further help or information.
Hoping God will give you both the strength to bear this loss and move ahead.