The specific cause for the pregnancy glow is not concrete, but there is solid educated explanations that suggest it is related to a couple of physiological changes that occur during pregnancy. As noted above, many of the skin changes are attributed to the influx of hormones. These hormones cause your glands to produce more oil, which makes your face more shiny.
Sources:(1994). Robert V. (Ed.), Mayo Clinic Complete Book of Pregnancy & Baby’s First Year (281). New York, NY: William Morrow and Company, Inc.Cunningham, F. Gary, Leveno, Kenneth J., et al (2005). Maternal Physiology in Williams Obstetrics 22nd edition (126). New York, NY: McGraw-Hill.