B’sha’ah tovah!
We hope your morning sickness passes soon.
Please check in with your doctor about the advisability of fasting. If there are no specific medical concerns, since your morning sickness is variable, you should start out fasting.
If you find yourself feeling sick on Tisha B’Av, then you may eat.
We recommend taking great care in the days before the fast to get well hydrated. It would also be wise to make plans for Tisha B’Av that would enable you to spend as much of the day as possible resting in a cool environment.
Please write back with any further questions.