What sweat does contain is small amounts of minerals and vitamins, so if you're purposely sweating profusely to get rid toxins you're likely only losing your prenatal vitamin. Calcium, sodium, magnesium, and other nutrients that are needed to build your baby's body and keep yours functioning properly are lost in sweat. Sweat is mainly water because the whole purpose is for the body to cool itself. Make sure if you're sweating that you're also drinking enough water so you avoid dehydration. This ironically will cause the toxins to be kept in the body because your kidneys don't have enough water to do their job. In addition, dehydration and loss of salt through sweat may increase chance of preeclampsia and low amniotic fluid.
I want to make sure to mention some cautions about detoxification in general for this time in life. Breastmilk can contain toxins that have been stored in your fat cells. This is why weight loss is suggested to only be 1 1/2 pounds a week at the most. We're all exposed to environmental toxins, so try to limit it where you can. Chemical cleaning products and pesticides shouldn't be handled by pregnant or breastfeeding women, or if absolutely necessary gloves should be used. Storing and reheating food in certain types of plastic is another source of toxins that is often overlooked.