SB 10.2.4-5

eke tam anurundhānā
 jñātayaḥ paryupāsate
hateṣu ṣaṭsu bāleṣu
 devakyā augraseninā
saptamo vaiṣṇavaṁ dhāma
 yam anantaṁ pracakṣate
garbho babhūva devakyā
 harṣa-śoka-vivardhanaḥ
Word for word: 
eke — some of them; tam — unto Kaṁsa; anurundhānāḥ — exactly following his policy; jñātayaḥ — relatives; paryupāsate — began to agree with him; hateṣu — having been killed; ṣaṭsu — six; bāleṣu — children; devakyāḥ — born of Devakī; augraseninā — by the son of Ugrasena (Kaṁsa); saptamaḥ — the seventh; vaiṣṇavam — of Lord Viṣṇu; dhāma — a plenary expansion; yam — unto whom; anantam — by the name Ananta; pracakṣate — is celebrated; garbhaḥ — embryo; babhūva — there was; devakyāḥ — of Devakī; harṣa-śoka-vivardhanaḥ — simultaneously arousing pleasure and lamentation.
Translation: 
Some of their relatives, however, began to follow Kaṁsa’s principles and act in his service. After Kaṁsa, the son of Ugrasena, killed the six sons of Devakī, a plenary portion of Kṛṣṇa entered her womb as her seventh child, arousing her pleasure and her lamentation. That plenary portion is celebrated by great sages as Ananta, who belongs to Kṛṣṇa’s second quadruple expansion.
Purport: 

Some of the chief devotees, such as Akrūra, stayed with Kaṁsa to satisfy him. This they did for various purposes. They all expected the Supreme Personality of Godhead to appear as the eighth child as soon as Devakī’s other children were killed by Kaṁsa, and they were eagerly awaiting His appearance. By remaining in Kaṁsa’s association, they would be able to see the Supreme Personality of Godhead take birth and display His childhood pastimes, and Akrūra would later go to Vṛndāvana to bring Kṛṣṇa and Balarāma to Mathurā. The word paryupāsate is significant because it indicates that some devotees wanted to stay near Kaṁsa in order to see all these pastimes of the Lord. The six children killed by Kaṁsa had formerly been sons of Marīci, but because of having been cursed by a brāhmaṇa, they were obliged to take birth as grandsons of Hiraṇyakaśipu. Kaṁsa had taken birth as Kālanemi, and now he was obliged to kill his own sons. This was a mystery. As soon as the sons of Devakī were killed, they would return to their original place. The devotees wanted to see this also. Generally speaking, no one kills his own nephews, but Kaṁsa was so cruel that he did so without hesitation. Ananta, Saṅkarṣaṇa, belongs to the second catur-vyūha, or quadruple expansion. This is the opinion of experienced commentators.