I believe that the word is talking about Paul's mother. That God, in his foreknowledge chose Paul, as an apostle and gave him the true meaning of the new covenant, which is the cross and to preach it to the gentiles. Remember Paul was a pharisee trained in the scriptures and actually persecuted the early church until his meeting on the road to Damascus. Many are called but few are chosen
That is the common response to that question but he was not. He was speaking of being born from this mother at time he identified when he mentioned it to the Corinthian church at 1 Corinthians 15:1-10
1 Corinthians 15:8 "And last of all he was seen of me also, as of one born out of due time."
1 Corinthians 15:9 "For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God."
That alerts us to look there in Galatians 1:15 to see if he indicates that to have been on the road to Damascus just as he related it to the Corinthians. And we see that he definitely did.
This happened to him at a time where he was, Galatians 1:14 "And profited in the Jews' religion above many my equals in mine own nation, being more exceedingly zealous of the traditions of my fathers." That was why he was so zealously persecuting the church.
Now notice: Galatians 1:15-16 "But when it pleased God, who separated me from my mother's womb, and called me by his grace, to reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood.."
The Jews viewed earthly Jerusalem as their mother in whose womb they were being nourished by the water of the word of the Law of God.
And we see that here, also:
Galatians 4:21 Tell me, ye that desire to be under the law, do ye not hear the law?
22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.
23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.
24 Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.
25 For
this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.
26 But Jerusalem which is above is free, which is the mother of us all.
27 For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband.
28
Now we, brethren, as Isaac was, are the children of promise.
29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.
30 Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman.
31
So then, brethren, we are not children of the bondwoman, but of the free.