Sleeping on a waterbed has been given mixed reviews by pregnancy experts. Some agree you should sleep on a regular bed during pregnancy, while others believe that a waterbed is perfectly safe for a pregnant woman.
One issue surrounds the electricity which is used to heat the water of waterbeds. A study in the 1980s conculded that there was a higher rate of miscarriage for women who slept on a waterbed (or used electric blankets) and those who did not. Other studies commissioned since then have found no relationship to miscarriage at all.
The other concern with waterbeds is the heat. If your body heat raises for prolonged periods of time to 101° or higher, it increases the likelyhood of miscarriage or neural tube birth defects. You should take care that your waterbed is not hot enough to raise your temperature, especially when combined with warm clothing and/or heavy blankets during the winter.
If you are concerned about your waterbed, perhaps it is time to think about purchasing a new bed. A baby should never be allowed on a waterbed due to the suffocation hazard (many babies have died this way) so if you would like to bring your baby to bed with you for nursing or sleeping, you should get your new regular bed now, rather than waiting until after the baby is born.