When should you consider getting a paternity test done, how far apart from when you had sex to the u/s date given from the doctor should you worry?

2 to 3 weeks: Keep the men at least 2 weeks apart, for ease of knowing who's the daddy. Otherwise, it may be too close to call, without doing DNA tests. The traditional rule is: the man married to the pregnant woman is the father of the baby. That has kept our civilization going for thousands of years. However, if one must know with more exactness, one can do DNA tests.