For the majority of the time, human pregnancies produce one baby per pregnancy period. That means that one ovum or egg from the mother has been fertilized by one sperm from the father. So the baby will receive half its DNA from the mother and the other half from the father.
However, some pregnancies will result in the birth of more than one baby, such as in the case of twins. This means that during the gestation period, the mother is carrying two growing infants at the same time in the same space. The exact reasons why twins are formed is unknown, but there are some factors that increase the chances, for example :
Certain ethnicities have higher chances of having identical twins, Black Africans have the highest while Asians have the lowest
If there are fraternal twins present in the family, the mother is more likely to produce fraternal twins, therefore it is hereditary
Becoming a mother around 30 or 40 years of age increases the chances of fraternal twins as women at that age are more likely to release multiple ova at the same time
Some procedures that are aimed at assisting fertilization may also produce twins, such as if the ovaries are stimulated in producing eggs and several are released at once or during IVF, in-vitro fertilization
There are two main ways for a mother to have twins. One way is if one ovum from the mother is fertilized by one sperm from the father and is then split in two producing the type of twins called monozygotic twins or identical twins. As is hinted in the name, these types of twins are genetically identical meaning that they share the exact same genetic information. This makes them compatible in the case of organ donors. If twins are identical they will always be the same sex, two girls or two boys. Unfortunately, this can also mean higher chances for the same complications, such as if one twin is diagnosed with an illness the other twin will most likely follow soon after.
The other way is if two separate eggs are fertilized by two separate sperm around the same time. The fertilized eggs then grow together in the same uterus. These are called dizygotic twins or fraternal twins and they are alike as any other two siblings, meaning that they share half of their DNA, making them less likely to be organ donors if ever needed. Fraternal twins can either be of the same sex or different sex and most often they look different then one another, but they may look similar as they are still siblings in any case.
It is possible for fraternal twins to have a different father, making them half siblings. This happens when two eggs from the same mother are fertilized by different fathers. The same logic that applies to siblings will apply here, they will share half of their DNA with the mother and the other half will depend on the father who fertilized the egg.
This may seem confusing when searching for the father of the child, but it is actually quite simple.
How can you be sure if the twin share the same Father ?
One option to check if the twins have the same father is by using a twin test. A sample is taken from both twins and the lab will test perform an analysis to see if they have the same DNA or not. This test will tell you if they are identical twins or fraternal, however it will not tell you if they share the same father. If they are identical, they share the same father, if they are fraternal, there’s a chance that they have different fathers.
The Paternity test can tell you for sure if the twins have the same father or not. Both twins and the father must participate in the test for it be carried out successfully.
A paternity test on twins can only be carried out after birth, so the prenatal paternity cannot be used in the case of twins. The reason for this is that the blood sample required for the prenatal test must contain the genetic sample from the mother and the genetic sample from one baby to be tested accurately. If more than one baby is present in the womb, there will be more than one genetic sample mixed with the mother’s blood and this will interfere with the analysis and provide inaccurate results.