This is called a vow and it is not recommended as the Prophet salla allahu alaihi wa sallam prohibited us from doing it and committing ourselves to something Allah didn’t oblige us to do!

Can we intend of performing 2 Rak’ah Nafil or Umra or Fast if our mission(specific neeyah) is accomplished or while in difficulty? Example: If I get pregnant or my mother recovers from ill health I’ll pray 10 Ra’kah Nawafil or perform Umra or Fast for 10 days