In any paternity proceeding, in the absence of a valid birth certificate indicating the birth weight, the mother shall be competent to testify as to the birth weight of the child whose paternity is at issue, and where the child whose paternity is at issue weighed 5 1/2 pounds or more at the time of its birth, the testimony of the mother as to the weight shall be presumptive evidence that the child was a full term child, unless competent evidence to the contrary is presented to the court. The conception of the child shall be presumed to have occurred within a span of time extending from 240 days to 300 days before the date of its birth, unless competent evidence to the contrary is presented to the court.