Its not. Its known to be a stimulant, and can induce abortion any time during pregnancy.
Also its contains strychnine- known poison which can be very fatal.
When most of the drugs we use everyday are considered harmful for the foetus during pregnancy, the effects of nux will be far more severe and dangerous. it is to be avoided. For what problem are you taking nux vomica anyway? That too in pregnancy. Use alternate drugs, atlest during prenancy for your medical problems.
Also check out this website-#mce_temp_url# ,where its mentioned that: Strychnine, one of the substances in the seeds of the nux vomica tree, is a poison that, in doses of 5 milligrams or more (as little as one seed), can cause anxiety, restlessness, painful convulsions of the body, breathing difficulties, and even death from suffocation or exhaustion. Long-term intake of even small amounts of strychnine can cause liver damage. This herb should be avoided, especially by women who are pregnant or breast-feeding.
Better be safe than sorry.