So, you're pregnant and not sure who your baby's father is?
Being pregnant and getting ready for a baby usually requires rather a lot of planning — expectant mothers need to decide where they are going to get their prenatal care, get all the baby stuff ready, figure out where they will give birth, how they are going to manage their new life as a mom, and what to name their baby. If you're not sure who the father is, however, you could be dealing with a whole other dimension of stress.
Do You Need A Paternity Test, Or Can Your Menstrual Cycle Come To The Rescue?
"Common wisdom" suggests that ovulation, the release of a mature egg from one of your ovaries, takes place around 14 days before your next menstrual period is due and that the four or five days before ovulation can be counted among the fertile window as sperm cells can survive for quite a while inside the female reproductive tract.
Research on the matter shows that the timing of ovulation is more varied than most people think, however. A study of nearly 700 menstrual cycles in over 200 healthy women showed that:
- Women have at least a 10 percent chance of being fertile between cycle days six and 21.
- Women occasionally ovulate much later than you might expect — between four and six percent of women sometimes have cycles in which they can conceive during the fifth week of their menstrual cycle.
- Only a third of women have fertile windows that neatly proceed along clinical guidelines, making them able to get pregnant only on cycle days 10 through 17. 
Those women who used ovulation tests to determine the timing of their surge in luteinizing hormone can be almost certain that they know when they ovulated . Those who used other methods to determine the timing of their ovulation, like salivary tests, basal body temperature, and cervical mucus monitoring, are less likely to be certain when they ovulated, as these methods will not always give you accurate information . Realistically, those women who have never tracked their ovulation have no idea when they were fertile during any one cycle, however.
This means, in short, that unless you've been using urinary ovulation tests, your menstrual cycle alone cannot give you a great deal of insight into the paternity of your baby if you are pregnant and unsure who the father is.
Pregnant And Not Sure Who The Father Is: Can Paternity Tests Be Done During Pregnancy?
For women who are pregnant and not sure who the father is, the 40-odd weeks of pregnancy can seem like an awfully long time to wait for answers. Thankfully, paternity testing can indeed be carried out during pregnancy.
Non-invasive prenatal paternity testing analyzes the fetal DNA that is found in maternal blood samples during pregnancy. This method, abbreviated as NIPP, requires only your blood and the DNA of the potential fathers.  It can be carried out any time after the tenth week of pregnancy.
Chorionic villus sampling, a prenatal test typically used to investigate potential chromosomal abnormalities, also gives you access to your baby's DNA. Chorionic villus sampling requires the removal of fetal cells from the placenta, either through the vagina or through the abdomen with a needle, and is usually carried out between 10 to 13 weeks gestation. It is important to be aware that CVS does carry a small risk of miscarriage with it. 
Amniocentesis, during which amniotic fluid is removed from your uterus through your abdomen with a needle, is another option for you if you want to find out who your baby's father is during pregnancy. Once again, it carries a small risk of miscarriage that decreases even further if your clinician is experienced, and amniocentesis is typically carried out between 11 and 20 weeks gestation. 
You can also wait until your child is born, in which case the paternity testing process poses no medical risks and doesn't even require blood to be drawn, as cheek swabs are usually used. Prenatal DNA testing is going to be more expensive than obtaining a paternity test after your baby is born.
Keep in mind that you do not need to pay for two separate DNA samples to be matched against your unborn child's DNA if you had two partners and simply want to know which one is the biological father of your baby for personal reasons. If the test result comes back negative for one of your partners, the other one must be the father. If you had more than two partners, matters become more complicated.