SB 10.1.58

Since the eighth son of Devakī was to kill Kaṁsa, one might ask what the need was for Vasudeva to deliver the first-born child. The answer is that Vasudeva had promised Kaṁsa that he would deliver all the children born of Devakī. Kaṁsa, being an asura, did not believe that the eighth child would kill him; he took it for granted that he might be killed by any of the children of Devakī. Vasudeva, therefore, to save Devakī, promised to give Kaṁsa every child, whether male or female. From another point of view, Vasudeva and Devakī were very pleased when they understood that the Supreme Personality of Godhead, Kṛṣṇa, would come as their eighth son. Vasudeva, a pure devotee of the Lord, was eager to see Kṛṣṇa appear as his child from the eighth pregnancy of Devakī. Therefore he wanted to deliver all the children quickly so that the eighth turn would come and Kṛṣṇa would appear. He begot one child every year so that Kṛṣṇa’s turn to appear would come as soon as possible.