My HCG at 16dpo with DD was 305 and with the twins at 16dpo was 351 - I never had any follow up BT's as my FS was happy with the first BT numbers. HCG numbers are not really a sign of multiples and having 2 embryo's put back obviously increases the chances but not always (DD's transfer was 2 embryo's and the the twins were 2 embryo's).

You will have to let us know what happens at her 7 week scan