Kira is the biological child of Sarah Manning. Yet we observe that at least two other clones (Beth Childs & Alison Hendrix) are unable to have children. The emphasis on this in two episodes, in regards to two different clones, now makes me suspicious that the references were not placed there by happenstance.
As far as I know, there is no scientific rule that clones are sterile, but the killer clone Helena (under the impression that she is the original) implies that the clones are poor copies of the original. Presumably, these "imperfections" could have resulted in the clones being sterile. Thus, since Sarah is the only clone so far that can have children, is it possible that she is the original from whom all the clones are derived.
As a side note, I don't want to get caught up on the order in which the clones were born, because I don't think this has much probative value. Presumably, they were all born around late March/early April of 1984. All the clones for whom we have information are born between March 10, 1984 and April 4, 1984, and 25 days seems like normal variance considering that some of the clones could have been premature and others overdue. I don't think birthdays will tell us anything.