There are prenatal paternity tests now, usually they can be done around 12-21 weeks (starting at 3-4 months.) It's still a good idea to a paternity test upon the child's birth as well.
In reference to the question, the way to tell who the father of your child is is to perform a paternity test after the birth. The baby's cheek cells will be gently collected with a sterile swab (I assure you the baby will not even feel it!). Then, if willing, any and all sex partners would have their cheek cells swabbed as well. The samples will be sent for a DNA comparison. It is 100% accurate and not at all invasive.
Please don't take this the wrong way, but the best way to "determine who the father is" is to only sleep with one man when you're not on Birth Control! Grrr ...
This is really a sad question, I am sure this is tearing you apart. You will need to do a DNA on the child immediately after its delivery. You don't want to wait too long, imagine trying to explain to your child that you had slept with different men and don't know who the father is!! This child needs to know as soon as its born, so does the Father. He needs to take responsibility for this child and so do you.
In response to the two above answers: And what if the person was raped? Education and the ability to share without judgments is much more useful then false morals. From what I know the test can only be taken after birth. I suggest you talk to your partner(s) and a gyn. to see what your options are.
Ignore self-righteous people. You are definitly not alone hon. I had an amniosintesis (totally not spelled right) done before I was 4 months pregnant to determine paternity. However, there is a small risk to baby. Also, if it is not already being done for medical reasons a doctor may opt no to do it and if he does, it can be spendy. I believe mine was 1300$. And keep in mind, no matter how hard it is to be unsure of who the father is, one this is for sure. YOU are the mother of lil one.
The father determines the sex.
The sex of a child is determined by the father because the genes that determine sex come from the male sperm.
Get a DNA test.
Both such twins obtain the half of genome which belongs to their father.
Get a DNA test.
The children of the father may be removed if they determine the father's drinking could cause the children harm.
just a swabbing the inside of your mouth will enable a lab to determine via DNA testing if you are the father.