SB 6.18.54

sāṁvatsaraṁ puṁsavanaṁ
 vratam etad aviplutam
dhārayiṣyasi cet tubhyaṁ
 śakra-hā bhavitā sutaḥ
sāṁvatsaram — for one year; puṁsavanam — called puṁsavana; vratam — vow; etat — this; aviplutam — without violation; dhārayiṣyasi — you will perform; cet — if; tubhyam — for you; śakra- — the killer of Indra; bhavitā — will be; sutaḥ — a son.
Kaśyapa Muni continued: If you perform this ceremony called puṁsavana, adhering to the vow with faith for at least one year, you will give birth to a son destined to kill Indra. But if there is any discrepancy in the discharge of this vow, the son will be a friend to Indra.