OH TIFF! Thats great news! Hmmm I thought I had posted but it must have got lost in the debugs earlier #-o I was going to say try not to worry as this happened to SIL at about the same time as you, she lost a cup of blood rushed to hosp (at this point they didn't know she had twins) then at the hosp they said she was fine but the woman didn't see two babies (silly) and she didn't find out till the following monday when she had her 12 wk scan booked and found out it was twin identical boys! Apparently bleeding is very common with twins, did you have anything like that with your first set? They told her the cervix and everything was closed so it wasn't coming from internally either.
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