Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.2.4

2.5.2.4

atha yadeṣa etaiścaturthe māsi yajate | tannāha nvevaitasya tathā prajā varuṇo gṛhṇātīti devā akurvanniti nvevaiṣa etatkaroti yāśca nvevāsya prajā jātā yāścājātāstā ubhayīrvaruṇapāśātpramuñcati tā asyānamīvā akilviṣāḥ prajāḥ prajāyate tasmādvā eṣa etaiścaturthe māsi yajate

Now when this (sacrificer) performs these offerings in the fourth month (after the Vaisvadeva), he does so either because thus Varuna does not seize his offspring, or because the gods performed (the same offering); and both the children that have been born to him and those that are yet unborn he thereby delivers from Varuna's noose, and his children are born without disease and blemish. This is why he performs these offerings in the fourth month.