The friends of the twice-born families are those who are born in the families of brahmanas, ksatriyas and Vaisyas, or the spiritually cultured families, but they themselves are not equal to their forefathers. Such descendants are not recognized as such for want of purificatory achievements. The purificatory activities begin even before the birth of a child, and the seed-giving reformatory process is called Garbhadhana-samskara.
One who has not undergone such Garbhadhana-samskara, or spiritual family planning, is not accepted as being of an actual twice-born family. The Garbhadhana-samskara is followed by other purificatory processes out of which the sacred thread ceremony is one. This is performed at the time of spiritual initiation.
After this particular samskara, one is rightly called twice-born. One birth is calculated during the seed-giving samskara, and the second birth is calculated at the time of spiritual initiation. One who has been able to undergo such important samskaras can be called a bona fide twice-born. SB 1.4.25
One who has not undergone such Garbhadhana-samskara, or spiritual family planning, is not accepted as being of an actual twice-born family. The Garbhadhana-samskara is followed by other purificatory processes out of which the sacred thread ceremony is one. This is performed at the time of spiritual initiation.
After this particular samskara, one is rightly called twice-born. One birth is calculated during the seed-giving samskara, and the second birth is calculated at the time of spiritual initiation. One who has been able to undergo such important samskaras can be called a bona fide twice-born. SB 1.4.25