I am having a second baby boy and I keep losing weight what can I do to keep my weight on and me and…

I am having a second baby boy and I keep losing weight what can I do to keep my weight on and me and my husband are getting everything together now for us and are baby boy and I have a one year old but he's with he's dad and I love my husband a lot and my baby boy but I keep losing weight and can't keep it on I'm 21weeks pregnant and I'm date October 17 the day before we get married