SB 3.31.1

śrī-bhagavān uvāca
karmaṇā daiva-netreṇa
 jantur dehopapattaye
striyāḥ praviṣṭa udaraṁ
 puṁso retaḥ-kaṇāśrayaḥ
Word for word: 
śrī-bhagavān uvāca — the Supreme Personality of Godhead said; karmaṇā — by the result of work; daiva-netreṇa — under the supervision of the Lord; jantuḥ — the living entity; deha — a body; upapattaye — for obtaining; striyāḥ — of a woman; praviṣṭaḥ — enters; udaram — the womb; puṁsaḥ — of a man; retaḥ — of semen; kaṇa — a particle; āśrayaḥ — dwelling in.
Translation: 
The Personality of Godhead said: Under the supervision of the Supreme Lord and according to the result of his work, the living entity, the soul, is made to enter into the womb of a woman through the particle of male semen to assume a particular type of body.
Purport: 

As stated in the last chapter, after suffering different kinds of hellish conditions, a man comes again to the human form of body. The same topic is continued in this chapter. In order to give a particular type of human form to a person who has already suffered hellish life, the soul is transferred to the semen of a man who is just suitable to become his father. During sexual intercourse, the soul is transferred through the semen of the father into the mother’s womb in order to produce a particular type of body. This process is applicable to all embodied living entities, but it is especially mentioned for the man who was transferred to the Andha-tāmisra hell. After suffering there, when he who has had many types of hellish bodies, like those of dogs and hogs, is to come again to the human form, he is given the chance to take his birth in the same type of body from which he degraded himself to hell.

Everything is done by the supervision of the Supreme Personality of Godhead. Material nature supplies the body, but it does so under the direction of the Supersoul. It is said in Bhagavad-gītā that a living entity is wandering in this material world on a chariot made by material nature. The Supreme Lord, as Supersoul, is always present with the individual soul. He directs material nature to supply a particular type of body to the individual soul according to the result of his work, and the material nature supplies it. Here one word, retaḥ-kaṇāśrayaḥ, is very significant because it indicates that it is not the semen of the man that creates life within the womb of a woman; rather, the living entity, the soul, takes shelter in a particle of semen and is then pushed into the womb of a woman. Then the body develops. There is no possibility of creating a living entity without the presence of the soul simply by sexual intercourse. The materialistic theory that there is no soul and that a child is born simply by material combination of the sperm and ovum is not very feasible. It is unacceptable.