The following post was written in 2001.  The inforamiton has not changed considerably, but here are some more recent articles on this topic:

New Data on Anxiety Medications and Pregnancy

SSRIs and Benzodiazepines: Reproductive Safety of Combined Treatment

The data regarding the use of the benzodiazepines (including Klonopin, Xanax, Ativan, and Valium) during pregnancy are somewhat controversial. Studies suggest there may be an increased risk of cleft lip and palate associated with first trimester exposure to these medications. This risk appears to be relatively small. Early reports suggest that the risk is about 0.7%; however, a recent study suggests that the risk may be even lower than that. Risk does not seem to be influenced by the dosage of medication taken by the other.

There are some concerns about the impact of benzodiazepines taken while pregnant on the newborn. As adults who take these medications may suffer from side effects, so too may the baby. Symptoms of toxicity have been reported in newborns, and these include sedation, decreased muscle tone (floppiness), and breathing problems. In general, these symptoms appear infrequently but probably occur more commonly in women taking higher dosages of medication. There are also some reports of benzodiazepine withdrawal occurring in newborns exposed to benzodiazepines in utero. Symptoms of benzodiazepine withdrawal include irritability, sleep disruption, and, less commonly, seizure.

For women who wish to stop anti-anxiety medications prior to pregnancy, it is best to do a slow taper (over 1 month) of the medication. This reduces the risk of withdrawal an/or recurrent symptoms. After stopping a medication, it may take several days or weeks to completely wash out the medication. Unfortunately, some women are unable to stop their anti-anxiety medication without significant symptoms and need to remain on a benzodiazepine during pregnancy. The decision to take anti-anxiety medications during pregnancy should be discussed with your psychiatrist or obstetrician, ideally before attempts to conceive.