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 anway? That too in pregnancy. Use alternate drugs, atlest during prenancy for your medical problems.