If you are eating that low cals then it technically is starving your body that's why you lose weight because you don't have enough calories to burn so your body burns itself up, if you are doing exercise too then you need to eat even more, 1500 is what they recommend to have to lose 1lb a week so it's not enough in pregnancy particularly if you are doing exercise because that uses extra
There will be a limit your body can deal with and lower than that it will store what you eat instead of using it because it doesn't know when it will next get enough cals if you get me