Praise be to Allaah.
It is permissible for a man to have intercourse with his pregnant wife whenever he wants, unless that will cause her harm, for it is haraam for him to do anything that will harm her. If it will not cause her harm but it is difficult for her, then it is better for him not to have intercourse with her, because avoiding things which are difficult for her is a kind of living with them honourably. Allaah says (interpretation of the meaning):
“and live with them honourably”
But it is haraam for a man to have intercourse with his wife when she is menstruating, or to have intercourse with her in her back passage, or to have intercourse with her when she is in nifaas (bleeding following childbirth). That is haraam and is not permitted, and a man must avoid that and do that which Allaah has permitted. If she is menstruating then he may enjoy her in other ways, apart from the vagina and back passage, because the Prophet SAWS (peace and blessings of Allaah be upon him) said: “Do whatever you like apart from intercourse.” (Narrated by Muslim, al-Hayd, 455).
A fatwa of Ibn ‘Uthaymeen. Fataawa al-‘Ulamaa’ fi ‘Ushrat al-Nisaa’, p. 55