it is very difficult to say the date of conception based on history and even online calculators. there are various tests which cane be done during pregancy: 1. Non-Invasive Prenatal Paternity (NIPP): is the most accurate non-invasive way to establish paternity before the baby is born. The process is state-of-the-art, combining the latest technology and proprietary methods of preserving and analyzing the baby’s DNA found naturally in the mother’s bloodstream. This test requires only a simple blood collection from the mother and alleged father and can be performed any time after the 8th week of pregnancy. The test is 99.9% accurate. 2. Amniocentesis: This test is performed in the second trimester,(4th-6th month) anywhere from the 14th-20th weeks of pregnancy. During this procedure, the doctor uses ultrasound to guide a thin needle into your uterus, through your abdomen. The needle draws out a small amount of amniotic fluid, which is tested. Risks include a small chance of harming the baby and miscarriage. Other side effects may include cramping, leaking of amniotic fluid, and vaginal bleeding. A doctor’s consent is needed to do this procedure for paternity testing. Chorionic Villus Sampling (CVS): This test consists of a thin needle or tube which a doctor inserts from the vagina, through the cervix, guided by an ultrasound, to obtain chorionic villi. Chorionic villi are little finger-like pieces of tissue attached to the wall of the uterus. The chorionic villi and the fetus come from the same fertilized egg, and have the same genetic makeup. This testing can be done earlier in pregnancy from the 10th-13th weeks. A doctor’s consent is needed to do this procedure for paternity testing.