Trolling, trolling, trolling on the river.
Having trouble with the meaning of 'prior', are you?
(g) a person born outside the geographical limits of the United States and its outlying possessions of parents one of whom is an alien, and the other a citizen of the United States who, prior to the birth of such person, was physically present in the United States or its outlying possessions for a period or periods totaling not less than five years at least two of which were after attaining the age of fourteen years... 8 U.S.C. Sec. 1401 (g) https://www.law.cornell.edu/uscode/text/8/1401
That's the current law; as noted above the earlier law required 10 years residency, at least 5 years after age 14, prior to the birth.
But I doubt you really care.
Birth Abroad to One Citizen and One Alien Parent in Wedlock
A child born abroad to one U.S. citizen parent and one alien parent acquires U.S. citizenship at birth under Section 301(g) of the INA provided the U.S. citizen parent was physically present in the United States or one of its outlying possessions for the time period required by the law applicable at the time of the child's birth. (For birth on or after November 14, 1986, a period of five years physical presence, two after the age of fourteen, is required. For birth between December 24, 1952 and November 13, 1986, a period of ten years, five after the age of fourteen, is required for physical presence in the United States or one of its outlying possessions to transmit U.S. citizenship to the child.) The U.S. citizen parent must be the genetic or the gestational parent and the legal parent of the child under local law at the time and place of the child’s birth to transmit U.S. citizenship.
Birth Abroad to One Citizen and One Alien Parent in Wedlock
There is no "prior to" in the section (g)s I have read. The law does say "acquires citizenship at birth" if those circumstances are met. My question has been all along if the mother and son moved back to the US when the son was age three, and both stayed, could the second five year residency be met after moving back to the US and citizenship recognized?