Would a husband be sinful if he prevented his pregnant wife from fasting the [first] ten [days of Dhul-Hijjah] out of fear of hardship for her?

There is no sin upon the husband for forbidding his wife from voluntary fasting, and it is not permissible for her to observe a voluntary fast except with his permission, due to the husband's right in this matter. The Shafi'is have made an exception for the fasts of the Day of Arafah and Ashura, allowing a woman to fast them without her husband's permission. The husband is advised to grant her permission if the fast does not harm her, so she may attain the reward of fasting on virtuous days.