IIf you stay on the Sandra route to the end, the MC winds up with both Haley and Sandra, whom he marries. If the MC gave Sandra his grandmother's ring, Sandra is pregnant. If the MC does not give Sandra the ring, he still marries Sandra and is still with Haley too, but nobody is pregnant. Although, Haley does get sick and hurls after drinking too many mimosas during a night out on the honeymoon. So, it is possible that Haley is pregnant, but not confirmed, because the game ends with Haley getting back into bed with Sandra and the MC right after throwing up.
There are other routes where Haley definitely becomes pregnant, and you certainly do NOT have to be with Sandra for there to be a good ending. However, it is my personal belief that the Haley + Sandra ending with Sandra pregnant is meant to be canon.