This is not recommended for use during pregnancy.

The manufacturer states:
During the treatment the water colour will change, this will happen with or without your feet present in the water. The colour change is mainly due to the reaction of the salt water with the low frequency current being transmitted by the coil or cartridge. However, you may very well notice fatty substances floating on top of the water which is generally lymphatic drainage. The treatment stimulates and opens the pores or sweat glands on your feet which can very often be blocked or, at best be very inefficient.