If it can be avoided, don't scan her in the first trimester (the first three months of pregnancy).
This is precautionary advice only. There is no good evidence that the mammalian embryo or foetus suffers any detrimental effect when subjected to static magnetic fields up to 4T. There is some equivocal data that suggest that the developing chick embryo is sensitive to prolonged exposure to weak gradient magnetic fields.
There is still the acoustic noise question. The development of hearing in the foetus occurs after 24 weeks and the noise of the scans might pose risk to the development of the hearing of the child. The mothers heart sounds are actually very loud already for the foetus because there is very good propagation of sound from the heart to the womb.