SB 10.5.23

diṣṭyā bhrātaḥ pravayasa
 idānīm aprajasya te
prajāśāyā nivṛttasya
 prajā yat samapadyata
Synonyms: 
diṣṭyā — it is by great fortune; bhrātaḥ — O my dear brother; pravayasaḥ — of you whose age is now quite advanced; idānīm — at the present moment; aprajasya — of one who did not have a son before; te — of you; prajā-āśāyāḥ nivṛttasya — of one who was almost hopeless of getting a son at this age; prajā — a son; yat — whatever; samapadyata — has been gotten by chance.
Translation: 
My dear brother Nanda Mahārāja, at an advanced age you had no son at all and were hopeless of having one. Therefore, that you now have a son is a sign of great fortune.
Purport: 

At an advanced age one generally cannot beget a male child. If by chance one does beget a child at this age, the child is generally female. Thus Vasudeva indirectly asked Nanda Mahārāja whether he had actually begotten a male child or a female child. Vasudeva knew that Yaśodā had given birth to a female child, whom he had stolen and replaced with a male child. This was a great mystery, and Vasudeva wanted to determine whether this mystery was already known to Nanda Mahārāja. On inquiring, however, he was confident that the mystery of Kṛṣṇa’s birth and His being placed in the care of Yaśodā was still hidden. There was no danger, since Kaṁsa at least could not learn what had already happened.