I have to second what SP said. I was very horny my first trimester and less as the pregnancy progressed. Every woman is different. Your job, dad, for the next nine months is to listen to her more than usual, pamper her more than usual, let her have her way most all of the time, and generally treat her even more like a queen than you did before she was pregnant. She's carrying the most precious gift anyone can give you, and it's not easy. The rest of her pregnancy should be all about her and her desires. In other words, don't pressure her about this, except maybe urge her to talk to her obstetrician.
There isn't a phycial reason I can think of why her vagina would be less deep during her first trimester because the uterus is still very, very small. Even so, most women are able to have sex with no great problems all the way to delivery. I remember my belly getting in the way more than having any vaginal problems.
The obvious thing to do is have her talk to her obstetrician about this. She'll get a pelvic exam anyway, and that's as good a time as any to bring up the sexual problems. It very well could be a problem, as being physically unable to accomodate your regular partner isn't normal, especially so early when very few structural changes have taken place.