وعليكم السلام ورحمة الله وبركاته In the Hadith of Ibn Abbas رضي الله عنه , the Prophet صلى الله عليه وسلم said : “If anyone of you, when having sexual intercourse with his wife, says : بِسْمِ الله اللّهُـمَّ جَنِّبْنا الشَّيْـطانَ، وَجَنِّبِ الشَّـيْطانَ ما رَزَقْـتَنا Bismillah, Allahumma jannibnish-Shaitan wa jannibish-Shaitana ma razaqtana, and if it is destined that they should have a child, then Satan will never be able to harm him.” ( Sahih Bukhari Volume 7, Book 62, Number 94 ) In our religion there is nothing called reciting in the heart . Every invocation has to be recited on the tongue with a voice. Therefore, this Dua has to be said out to the extent that the person moves his tongue with it or hear it for himself. The Prophet صلى الله عليه وسلم only told us to recite this Dua before intercourse whether she is in a condition of pregnancy or not. But if some one were to ask what is the benefit of saying this Dua while doing intercourse during her pregnancy , giving the reason that this Dua is only for the child , we answer that there are many reasons and one of them is sticking and adhering to the Sunnah of the Prophet صلى الله عليه وسلم and getting the reward for this. And Allah عز وجل knows best .