Before anyone starts getting nervous, I would like to clarify that no, I am NOT pregnant! However, since I have been asked about the safety of pregnant investigators several times, and had to deal with the issue myself when I was pregnant with Luke, I thought I'd share my thoughts here. This is an opinion piece, and doesn't necessarily reflect the views of other paranormal investigators. This was taken from a message board response, so that's why it reads a little informally.
Although it is a decision that an individual has to make for herself, I investigated throughout my pregnancy, and my son turned out fine. In fact, pregnant women are often an asset to an investigation. As part of the whole survival of the species thing, pregnant women generally will notice a heightened sense of smell, become more observant, and increase their intuition. These characteristics that are designed to ensure the safety of the unborn fetus from outside sources can generally be an asset to investigations.
More on the theoretical side, a pregnant woman may actually even attract more paranormal activity, either because she is seen as a great source of energy, with all those hormones spreading out like a beacon...or simply because of the maternal bond. There is lots of anecdotal evidence that certain entities, such as those possibly of young children, who seem to cling to mommy figures...and what is more apparent as a mommy figure than someone who is visibly pregnant? Unfortunately, those same hormones and survival characteristics COULD lead to false positives, and in extremely rare cases, may even result in cases of recurrent spontaneous psychokinesis. RSPK, generally thought to be the source of poltergeist activity, is often attributed to a young person on the cusp of puberty. However, pregnant woman, and even menopausal women, are going through similar hormonal and life changes that can have a similar effect.
Anyway, one of the biggest concerns people tend to have is that they are worried that something will "attach" to the unborn child. I'm not saying it COULDN'T happen, even though I have NEVER heard of such a case, but it would be extremely unlikely that anything encountered out in the field could be capable of doing something like that. Most cases are either not paranormally caused to begin with, or are residual.
If you follow the work of Dr. (well, former, since her license was yanked) Edith Fiore, you're WAY more likely to have something attach to the baby at the hospital during the delivery, since there in an abundance of lost souls wandering around confused. Due to an incompetent cervix, I ended up in the hospital for quite awhile. The hospital I stayed at already had a reputation for being haunted, and the baby next to my son in the NICU did pass away, but again, my son doesn't appear to be having any adverse affects. In fact, he's a little abnormal when it comes to the ghost stuff--a common belief is that new babies and pregnant women attract the attention of ghosts and spirits, but all activity in my house died completely out for the first 6 months he was home, lol. (According to a local psychic who "read" my son when he was a few weeks old, my grandfather's spirit moved in when my son came home, and forced all the other paranormal activity out of the home.)
Having said all that, I don't necessarily recommend pregnant women to do field investigations for more practical reasons. Many places that we go to investigate have minor safety issues that could become big safety issues for someone who is pregnant. When you're pregnant, most people's body changes so much, that they don't have the same control over it as they did before. Some drop things...all the time, lol. A mis-step on an uneven staircase can lead to disaster. Some locations have mold, high levels of EMF, clients who may not be the most mentally stable--all issues that are not ideal conditions for a pregnant woman. And...no one wants to be on location with limited bathroom facilities and have to go pee every five minutes (speaking from experience, here).
Again, choosing to investigate at this time in one's life is a very personal decision, but I recommend that anyone who has ANY reservations at all refrain from actively investigating. It is better to be safe than sorry, and not have the anxiety that if anything weird does happen, it wasn't the result of going on an investigation.
However, if one chooses not to actively investigate or do field research during the pregnancy, that doesn't mean being completely left out of the investigation process; there are plenty of other things that can be done besides actually go out in the field. This is a perfect opportunity to help with evidence review, website maintenance if you are on a team with a site, research possible locations, learn as much about the field as possible, and train yourself on basic equipment. If possible, take a photography class, or study up on how different equipment works. Make calls to locations about possible investigations, and continue to participate in any group meetings. Before long, these particular limitations will be over, and you'll have a new member of the next generation of paranormal investigators!