Baby B measuring 1 week behind

Yesterday I had a ultrasound with my high risk doctor. I honestly don’t even know how far along I am because I never had a period after coming off my birth control.. from the scan they had 15weeks 6 days I think. Well They did the scan A is only 2 days behind and baby B is a week behind but is weighing the same as her sister. All her legs arms brain stomach and other areas look good. They’re concerned and basically telling me it could be abnormalities or her placenta won’t know until 20 weeks when we can run a different test. They were first called Mo/di twins but now they have them as di/di twins because they found two placentas and they have a membrane separating the two of them. My high risk dr said I’ll most likely lose baby B I’m so upset because I feel like I have already failed my girls. I go back on the 13th and I’m just hoping a praying baby B is only at least a few days behind instead of a week. Has any one else had one of their babies measure a week behind and still had both their twins at delivery