Lifting in the first trimester...?

i'm around six weeks pregnant. my work entails carrying boxes between 1 and 30 (sometimes a little more than 30) lbs from a truck to a designated area on the floor. then from the floor to my work station. on busy days i'll have to do around 40 to 50 boxes and on really slow days we might get in 10 boxes. the busy days are only the first full week of the month (5 days; m-f) and the rest of the month goes down in number of boxes. what is a safe weight to lift? also, my work does provide dollies and carts which have wheels. but both still require me to do some lifting.