There is a popular story about why twins were born to Sumitra. Dasaratha, despite having three wives had no children and none of the queens had been able to give him an heir to the Ikshvaku dynasty. The thought of his dynasty ending with him saddened Dasaratha and he was very eager to have a son who would take care of the throne of Ayodhya.
GuruVasistha who was the raj (royal) purohit of Ayodhya asked the King the reason for his sadness and the King conveyed his worries about the lack of a male heir to his dynasty. On Vasistha's request, Dasharatha did a Yagya at the end of which he gave a plate of sweets each to Kausalya and Kaikeyi and asked them to share their plates with the third queen Sumitra. Sumitra got one bite from Kausalya's plate and one from Kaikeyi's and thus gave birth to twins. With Kausalya and Kaikeyi each bearing one son. Sumitra’s sons were later named by guru Vasistha as Lakshmana and Shatrughana.