Hello Thanks for writing to us with your health concern. It has already been possible to determine paternity during pregnancy using amniocentesis or chorionic villus sampling, the same medical procedures used to test a fetus for Down syndrome. But those procedures are invasive and carry a small risk of inducing a miscarriage, so they are rarely used for paternity testing. By contrast, the new tests require only blood samples from the pregnant woman and the potential father. These tests like Harmony / Natera / panorama are almost 99 % reliable in diagnosing the paternity. They are expensive though. Based on ultrasound scan or clinical judgment, the two episodes of intercourse are too close by ( only 5 days apart ). So you can go for the non invasive testing anytime after the eighth week of pregnancy. Please get it done to avoid a lifetime of uncertainty and stress. All the best Take care.