Staying in a hot tub for longer than 10 to 20 minutes at a time could cause your body to overheat especially if you're pregnant.
When you sit in a hot tub too long and your body overheats it could cause dizziness, fainting, nausea and other heat related illness.
So only stay in the Hot Tub for at most 20 minutes at a time and then get out of the hot tub for awhile to cool off and then return to the hot tub again.