While most doctors advise against it, some doctors still will allow their patients to tan during pregnancy.
Here is a great article that discusses the subject even more: Pregnancy and Tanning.

What type of sunless tanning
do you recommend for expecting Moms?

Sunless tanning creams: We have heard many people, pregnant and non-pregnant alike, complain about the sunless tanners. Because of the inconsistency in the color and the inability to evenly apply them, we do not recommend using them.
Mystic Tanning Booth: While the Mystic systems offer you privacy, they do not offer you consistent color and it is even harder to move around in them. Some of our clients have told us that they feel closterphobic in the booths. Definitely not what you are looking for as an expecting mother.
Sunless airbrush tanning: You have control of what parts of your body you want sprayed. The airbrush tanning will give you even, natural UV-free color and eliminate streakiness.
Our system: Unlike a lot of airbrush equipment and the spray tan booths, our unit has very little overspray or fumes from the solution at all. It transfers 85% of the solution onto the client, compared to just 30% transfer from other airbrush systems. This means more solution on you, less in the air to breathe in. In fact, our system is so efficient you will be dry by the time we are done spraying you.