If I remember correctly it's pretty well documented how much more you need per day when pregnant. In the first months it's minimal (definitely not 300-500kcal), and progresses to still-not-much right before giving birth (I think still not 500kcal). Remember, the baby weighs just a few kilos at birth. You need energy to grow that, sure, but that won't be very much. It's not like you have to "eat for 2" literally, not even close.