Srimati Rohini came to Gokula from Mathura at the beginning of the month of Sravana. At that time, she was pregnant for about three months.
Later, when Devaki was pregnant after several months, the 7th child of Devaki was transferred by the power of Yogamaya to the womb of Mother Rohini – just as Sri Vasudeva has brought Lord Krsna from Mathura to Gokula. At that time Lord Krsna, who was brought by Vasudeva, entered into baby Krsna, who was already in Gokula born from Mother Yasoda. They became one entity, one Krsna.
In the same way, Balarama, who was in the womb of Devaki, was taken out by Yogamaya and He entered the womb of Mother Rohini where Balarama was already there, in Rohini’s seventh month of her pregnancy. In this way, both Balaramas merge together and became one also.
At that time the news spread that there was a miscarriage of Mother Devaki. They didn’t understand that the child in her womb had been transferred to the womb of Mother Rohini.
Lord Baladeva remained in the womb of Rohini for 16 months.
Just a little before the birth of Lord Krsna, Lord Balarama was born of Mother Rohini on the Purnima day of the month of Sravana, after Rohini had been pregnant for 16 months.