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.
First of all, you should know that it is not permissible to accuse your wife of having committed adultery without clear and confirmed evidence. In principle, a Muslim is innocent until he is proven guilty. Allaah Says (what means): {O you who have believed if there comes to you a disobedient one with information, investigate, lest you harm a people out of ignorance and become, over what you have done, regretful.}[Quran 49:6] Allaah also Says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin}[Quran 49:12] What people say, or the presence of the telephone number of that man with her and so forth, is not enough evidence to accuse her. For more benefit, please refer to Fataawa 92038 and 87877.
Similarly, the fetus in her womb is your child because you are the owner of the bed, i.e. her husband. Indeed, it is confirmed that the Prophet said: "The child is for (i.e. traced back to) the owner of the bed (i.e. the legitimate husband)." [Al-Bukhari and Muslim] It is only with Li’aan that you may deny this child. [Li’aan means when you and your wife invoke the curse of Allaah on the liar amongst you when you accuse her of having committed adultery]
On the other hand, there is no doubt that your wife is sinful if she let that man to enter your home when you were absent. It is confirmed that the Prophet said: "It is your right over them [i.e. your wives] that they do not let anyone whom you dislike tread on your beds.” [Muslim] Also, this is considered as disobedience from her and the Sharee’ah clarified how to deal with a disobedient wife in Fatwa 85402, so please refer to it.
However, if she repents and returns to Allaah and she becomes righteous, then praise be to Allaah and you should keep her and keep good company with her. We advise you to be keen on staying in your country in order to keep yourself and your wife chaste and nurture your children in a good manner, especially that there are many disadvantages in residing in non-Muslim countries. Moreover, you should know that it is not permissible for a husband to be absent from his wife more than six months except with her permission as we clarified in Fataawa 85711 and 84073.
If she does not become righteous or that you hate to live with her, and you fear that this will lead you to be unjust to her, then it is more appropriate to divorce her. Allaah Says (what means): {But if they separate [by divorce], Allaah will enrich each [of them] from His abundance. And ever is Allaah Encompassing and Wise.}[Quran 4:130]
Allaah Knows best.