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That means you conceived around July 31. The accuracy of the ultrasound is plus minus 7 days. It is unlikely the baby is the Hispanic guy's baby. Reassured.
Also your doctor's saying about the heartbeats is correct. You don't need to worry about it. Reassured.
You can use a prenatal paternity test with your blood. http://www.prenatalgeneticscenter.com/services/prenatal-dna-paternity-test/
The conception occurs pregnant weeks minus 2 weeks before. So in your case, the conception occurred 4 weeks before 8/20/15. Sorry the conception date is July 23 from the calculaton. But your doctor should have done ultrasound and you were 36 weeks 4 days on March 16. The accuracy is plus minus 3 days. You conceived 34 weeks 4 days ago. Your conception date is July 18 plus minus 3 days. It is between July 15 and July 21. Even though the accuracy is plus minus 7 days, it is between July 11 and July 25. It is impossible the hispanic guy is your baby's father.
It is not blood type but DNA. Parentage testing uses tandemly repeated DNA length polymorphisms to compare DNA samples between individuals. It takes long time to explain about the DNA test to you.
Sorry correction. It is DNA test after birth. Cell-free fetal DNA isolated from the plasma of the mother’s blood is analyzed using a dense microarray chip that examines 317,000 genetic markers known as single nucleotide polymorphisms (SNPs). An informatics algorithm is used to compute the similarity of genetic markers between the fetal DNA and the alleged father’s DNA, as well as to unrelated random individuals. It costs $1625 in the United States.