Sex has been tested and re-tested over and over and has not been found to interfere with pregnancy. There have actually been tests done involving monitoring the fetal heart rate during orgasm, and all was well. Of course, I'm stunned at how any researcher could talk anyone into such a monitored experiment. In my practice, I recommend stopping intercourse only when it becomes uncomfortable for either partner. Certainly the swelling of the delicate vaginal mucosa can make the mechanical act of intercourse irritating and painful.
Vaginal dryness is probably a result of the swelling mentioned above.
Pregnancy makes a woman more prone to yeast infections , which once again cause swelling, but also inflammation, of the vaginal tissue. If it doesn't, feel free.