I have studied in our texts that a soul will have the memory of its previous births after death, and before birth. My question is when does the soul lose its memory when entering into a body for a birth?
According to the Garbha Upanishad, the soul can remember its past life when it is 9 months old in the womb:
Whatever is consumed or drunk by the mother passes through the nerves and vessels to the child, becoming the source of his satisfaction. During the ninth month, all outer signs attain completeness. And he is reminded of his previous birth, and recounts the good and bad deeds committed.
This text describes how the foetus is gradually developed over the months in details.
In the 8th month only the foetus gets the status of a full fledged Jiva which is aware of it's own existence.
And from the 9th month onwards, the Jiva contemplates on it's situation. It resolves again and again that if it comes out through that narrow passage of the mother then it will do every possible efforts to make this life it's last life. He resolves thus:
He thinks: I have seen thousands of wombs, eaten several kinds of food and sucked many breasts. Born and dead again and again, I am immersed in grief but see no remedy. Thinking of my good and bad deeds, I am suffering alone, although the bodies that enjoyed the fruits are gone.
When I get out of this womb, I will take refuge in Sā¡khya-Yoga, which destroys misery and yields liberation; when I get out of this womb, I will take refuge in Maheśvara, who destroys misery and grants liberation.
When I get out of this womb, I will take refuge in Nārāyaªa, who destroys misery and grants liberation. When I get out of this womb, I will meditate on the eternal Brahman.
But as soon as it comes out of the mother's womb through the passage it forgets all of that, due to the influence of Maya and due to the force of the Suti Vayu:
When he reaches the birth canal and comes out of it with great difficulty, he is touched by an all-pervading movement [Māyā] that causes him to forget previous births and the good and the bad deeds performed therein.
So, as soon as it comes out of the mother's womb --- is the answer.
The Tantrik compendium ShAtkAnanda Taragini, in it's 1st Chapter, quotes the following verses from Tantras (I'm only giving here the few verses that are relevant to the current topic):
Navame mAsi garbhasthah sarvAn samsmarate hridA |
NavadwAre pure dehi samayAmshcha vikkArAn ||
During the 9th month [in the womb] the Jiva remembers everything. And, being situated in the body endowed with the 9 gates, thinks about the merits and demerits of the Karmas (or conducts).
But, then when it comes out of it's mother's womb, due to pressure and pain of coming out of such a narrow passage, as well as due to the forceful effect of the Suti VAyu, it forgets everything.
Navame dashame mAsi pravalaih suuti mArutaih ||
NihsAryate vAna iva jantus chidrena sajjvarah |
Patitohapi na jAnAti murchitohapi tatas chyutim ||
SutivAtasya vegena yonirandhrasya piranAt |
Vismritam jyAnam garbhe yat chintitam hrudi ||
YathA bhavati tattveshu sutibhuteshu pirnAt |
MAtaram smarati nityam bhubhukshA drira rodanam ||
And, in the 9th or in the 10th month, the kid, being affected with severe pain, and by the forceful effect of the Suti VAyu, comes out of that narrow channel like an arrow. Since it gets immediately unconscious after coming out of the womb it does not even realise that it is separated from the mother. Due to the force of the Suti VAyu and due to the pain and pressure it suffers while coming out of the narrow passage, it forgets everything - the memory of past life as well all the reslutions that it had taken. The kid being inflicted by the shaft like Suti VAyu cries in hunger and always looks for (or remembers) it's mother.
According to KAlikA PurAna (KP), it's MahAmAyA (the Great Enchantress) who is responsible for all this. We have the following verses here:
GarbhAntarjyAn sampannam preritam suti mArutai |
Utpannam jyAnarahitam kurute yA nirantaram ||
PurvAti purva samskAra sammoham samniyojya cha |
AhArArdyau tato moham mamatvam jyAn samshayam ||
KrodhoparodhanAdishu khsiptvA khsiptvA punah punah |
PashchAt kAme niyojyAshu chintAtyuktam aharnisham ||
Within the womb the Jiva has full knowledge; but when directed by the Suti VAyu the Jiva gets born, she (or MahAmAyA) makes it devoid of that knowledge. And gets it into the SamsAra again and again, which is full of delusion, compassion, hunger, anger, desires etc etc..
(KP 6- 61,62,53)
I am answering this question acording to the Garuda Purana
Jiva loses the memory of its previous births at the moment he comes out of the mother's womb.
It has been mentioned multiple times in the Garuda Purana.
The following is the statement made by the lord Vishnu in conversation with Garuda stating that the human forgets the knowledge of previous births instantly after leaving the womb of its mother.
When he takes birth as a human being the sins visit him. When he remembers his past sinful life he becomes sorrowful. When he resides in human womb after sojourning in the various beings such as birds, etc., the full velocity of his past actions forces an impact on him. Only after residing in the womb does he realise it. He begins to realize the tortures of life, such as anxiety, sicknes.s, distress, old age, deformity and the rest. But the moment he comes out of the womb, ignorance envelops him. As an infant he realizes nothing.
The following is also the similar statement of Vishnu stating the same
In the womb he recollects the past events and happenings of worldly affairs. Tormented from all sides he feels depressed ‘‘As soon as I leave the womb I shall be very cautious. I shall never repeat mistakes. I shall so act that I may not be born again.” The living being thinks thus and further recollects events of hundreds of past births-events which accrued to him from causes known and unknown. Then at the scheduled time the living being emerges from the embryo with the head downward in the ninth or tenth month. While leaving the womb he suffers and faints. When he comes out of the womb he cries in anguish. Then with the touch of air he obtains little relief and comes to consciousness. Then the hypnotising power of Visnu overpowers him. Deluded by that power he loses power of discernment.