Srimad Bhagavatam Canto 10, Chapter 01, Text 08

SB 10.1.8

rohinyas tanayah prokto
ramah sankarsanas tvaya
devakya garbha-sambandhah
kuto dehantaram vina
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
My dear Sukadeva Gosvami, you have already explained that Sankarsana, who belongs to the second quadruple, appeared as the son of Rohini named Balarama. If Balarama was not transferred from one body to another, how is it possible that He was first in the womb of Devaki and then in the womb of Rohini? Kindly explain this to me.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Here is a question particularly directed at understanding Balarama, who is Sankarsana Himself. Balarama is well known as the son of Rohini, yet it is also known that He was the son of Devaki. Pariksit Maharaja wanted to understand the mystery of Balarama’s being the son of both Devaki and Rohini.
Srimad Bhagavatam Canto 10, Chapter 01, Text 05-07
Srimad Bhagavatam Canto 10, Chapter 01, Text 09