I'm a bigger girl and with both my pregnancies i lost weight. But my dr was okay with that because I was losing fat and gaining baby. Once I hit 6 months i stayed the exact same weight. My Dr said (for me) 12-17lbs was an ok gain
Im a size 22 & my dr says bigger girls dont usually put on alot of weight, maybe 7-15 lbs; whereas smaller people put on 20-50lbs. Im 15w2d and have actually lost 7lbs since i found out i was preggo at 6w4d. Dr says its nothing to worry about, i think the weight loss is because i decided to try and eat healthier but it also helps that im craving fruits and veggies. I dtill recommend speaking to your dr.
You should discuss this with your doctor as they will know your medical history and be able to monitor you and the baby. Im in similar situation and am just going to let them worry about it! :-)