underweight?

Can being underweight cause any health problems to the baby? or possibly loss? I'm 5'2 and weigh atleast 100pds. Before I became pregnant I weighed in at 110 but during my 4-5 weeks I had nausea and vomiting to extreme and dropped those 10 pounds. now I find it hard to gain that weight back. Any tips? Im currently 12 wks pregnant.