He is talking to people that have already been born from a mother. He isn't talking to unborn people. You must be born again. Water baptism, Spirit Holy Spirit.
John 3:5-7
5 Jesus answered, "Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God.
6 "That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.
7 "Do not marvel that I said to you, 'You must be born again.'
(NKJ)
But Jesus didn't mention anything about being baptized in water to Nicodemus when chiding him for not remembering "that little detail". So if baptizing for salvation could have been known by reading the OT, doncha think Nicodemus, a teacher of the Jews, would not have needed to ask the question??
"Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?" John 3:10 (KJV)
[Acts 2:38
38 Then Peter said to them, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit.
(NKJ)
You have water baptism as being a part of a very legalistic manner in which people can only be saved. What do you think happened when Jesus said to the thief on the cross,"This day you shall be with Me in Paradise". How could that be without water baptism?
Note: Acts 2:38 was NOT for salvation in the sense it has been given us to understand.. These Jews were righteous Jews who had now learned from Peter that Jesus had made peace with God in their behalf. Peter was in effect saying, 'change your thinking about your own righteousness in believing it to be sufficient for your salvation'. This is not to be confused with the derogatory accusation of being "self-righteousness" but having an honest appraisal of themselves according to the law not unlike how Paul thought of himself when persecuting the church, that he could say,
". . . as to righteousness under the law, blameless". However, whatever gain I had, I counted as loss for the sake of Christ."Philippians 3:7 (ESV)