My wife said to me, "I vow to slaughter a sheep if my pregnancy goes well," and I encouraged her. Does my encouragement for her vow constitute a binding vow on my part as well?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

If you mean that your wife told you that she vowed to slaughter a sheep if her pregnancy went well and you approved of her vow, then this constitutes a valid, binding vow for her. It is incumbent on her to deliver on her vow when the thing upon which she rendered her vow conditional takes place, i.e. if her pregnancy goes well (and she gives birth to her child). Allaah, the Exalted, commands us to fulfill our vows, as He says (what means): {(Then let them …) and fulfill their vows...} [Quran 22:29] Moreover, the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vows to obey Allaah must obey Him..." [Al-Bukhari and others]

As for you, when you encourage your wife or consent to her vow, then this does not constitute a valid, binding vow on your part. However, if you mean that she took a vow that you would slaughter a sheep if the pregnancy went well, then your wife is not obliged to do anything because she had vowed something that she is not entitled to fulfill.

The Prophet, sallallaahu ‘alayhi wa sallam, said: "There is no vow to commit disobedience, and no vow should be made concerning that which a man does not possess." [Muslim]

If you uttered any statement suggesting commitment, it would be considered a binding vow that you are obliged to fulfill on your part. However, if your statement did not suggest commitment, then it is not incumbent on you to do anything in this regard.

Allaah Knows best.