This from a midwife on a parenting site....
Many dermatologists tell us that there is no such thing as a "safe, good or healthy tan." Numerous studies have demonstrated that exposure to ultraviolet radiation (UVR even at levels attained in an average tanning salon treatment) is hazardous. Repeated exposure to UVR leads to tanning with harmful cumulative effects. Darkening of the skin represents a defense mechanism against recurrent damage from UVR. Although the ability to tan can be protective, tanned skin ages prematurely and has an increased risk of developing skin cancer.
According to a National Institute of Health (NIH) document Solar Radiation and Exposure to Sunlamps and Sunbeds: "Most tanning bulbs sold in the United States for use in sunbeds emit 'substantial doses of both UVB and UVA'(Swerdlow and Weinstock, 1998, citing 'personal communication from industry sources.' Many of the home and salon devices in the 1980s emitted both UVA and UVB radiation, but current devices emit predominantly UVA (FTC,1997; Sikes,1998)."
Some authorities are studying a potential connection between neural tube defects and ultraviolet radiation. They site evidence that shows that the radiation breaks down folate and puts preconceptional and early pregnant women at higher risk for giving birth to a baby with spinal bifida or other types of neural tube defects. One such article, published in 1999 by Jablonski in Medical Hypothesis, concluded that "intense or prolonged periconceptual exposure of women to UV light for recreational or therapeutic reasons should be avoided."
The British Medical Journal in 1997 (Volume 314(7089) published an article by Wright et al. which stated "there can be no doubt that sunbeds are widely used and that they carry a risk of inducing skin cancer. Clearly the higher the ultraviolet light output the greater the risk."
There have been no controlled studies of the direct effects of tanning or tanning beds on pregnant women or their unborn child. In view of the evidence of damage from ultraviolet radiation to the skin of the mother, however, it makes sense to me to avoid this type of activity for the duration of pregnancy.