Is 13 days enough time span to determine paternity LMP 3/28/14 regular cycle u/s on 6/3 for 8w1d & on 6/16 for 9w5d (different place) sex 4/3 4/16-24?
No. In order to determine paternity, you would need DNA sample for the child (not likely until the child is born) and from the person you suspect is the father. I do not know of a way to test prior to this. The roughly 2 weeks difference in sex with different partners (?) also makes it too close to call. Your US dates are not exact and have error of up to about a week at that gestation. Sorry;
See Below... A Crown-Rump length measurement between 7-10 weeks is accurate to +/- 3 days if it is done by a well trained sonographer on modern equipment. The due dates from your two ultrasounds are in this range. Paternity is a sticky. It would be wise to get paternity testing done after birth. Sometimes a blood test of baby and dad is all that is needed. Other times its more complicated. Best wishes.
Not by US. I think this is going to require genetic testing or blood work compatibility testing. The times are within the sensitivity of the ultrasound dating I am afraid.