The pregnancy in a human lasts for 40 weeks but it might last for both a shorter or a longer period of time.
The important fact is that the ovulation and the conception are possible exactly fourteen days before the start of the next expected menstrual period.
Given the data you provided (last menstrual period on 12/27/2008 and 28 days average length of menstrual cycle) your next menstrual period should have started on 01/23/2009 if you did not conceive. When you subtract fourteen days you will get the most probable date of ovulation which is 01/13/2009.
However, this is correct only if your menstrual cycle was supposed to be the same 28 days in length as usual. There is no way that you can know this fact for sure. Therefore, a paternity test is the only accurate test that can tell who is the father of the child.
The paternity test usually uses a buccal swab, but other materials containing fluids and tissue might be used as well, like: blood, saliva, semen or even tissue on a toothbrush, razor etc.
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