Ob-Gyn questions? Ask a gynecologist online.
HCG-level has a very wide range (7th week 1000-50.000), so this doesn´t allow a secure statement. But the first ultrasound measuring leads to a conception date at Jan.5th, the second at Jan.13th. Therefore it´s pretty sure that you can exclude the first partner as father.
Dr. N. Scheufele
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Thanks a lot, all the best for you,