There is no problem doing MR on pregnant women at any stage of preagnancy, but for ass-covering reasons, we try not to image these women until after organogenesis (after 1st trimester). If it is a life-or-death situation, we might do it, but we have not seen such a case, ever.
Avoid gadolinium based contrast agents. A study has been reported as showing that they can cross the placenta in primates and appear in the fetal bladder within minutes of administration.