Or HaChaim on Genesis 25:21/אור החיים על בראשית כ״ה:כ״א

1 ויעתר לו, "He responded to his entreaty." The reason that the Torah does not mention the customary "Isaac knew his wife" (as in Genesis 4,1, or Samuel I 1,19) prior to reporting her pregnancy may have been that they prayed within three days of having had marital relations. Their prayer was that the semen should not go to waste (compare Berachot 60 on the subject of semen turning unfit for conception).