Narayana enters the human body through food.This food becomes retas in the MAN . This retas is the essence of the man arising out of all of his limbs. It is held within oneself.
When this retas is sprinkled in a woman ,It is the first birth.This becomes a part of the woman as her other limbs .hence She should not be troubled by the husband. Woman should take care of her womb as the presence of divine in her.A woman who thinks thus ,should be taken care of.He who respects the mother and the son in the womb ,worships God present in the womb obtains all the wishes.
The birth of the son is the second birth from the womb onto the earth.The God in the son remains on the earth for sometime doing karma on behalf of the father. Then the God present in the father leaves him along with the jeeva and attains urdhwa loka.
When he is born in another place ,it is the third birth. Thus Vishnu protects the jeeva all through his births.
Note : When a man leaves his retas in a woman , a part of him enters the woman , woman carries her husband's amsha. This is pratham janma . When woman gives birth , son born though a different jeeva also carries his father's amsha . Thus child is the father. This dwitiya janma . Here this amsha remains in the son and it is because of this amsha , whatever man does karma it is also distributed or experienced by the father. When father leaves this world , his amsha leaves with him ,remains with him in all the swarga and other lokas.When he is born elsewhere it is the third janma . But earlier amshas continue to reap benefits of the karma ,Thus god enables one to reap the karma through progeny in all the janma and runanubandha remains intact even if he is born elsewhere. The shraddha that he performs reaches him though he is born elsewhere and is unconnected physically ,his amsha stays connected with the help of Pitru devatas.