John Gill Commentary Genesis 25:24

"And when her days to be delivered were fulfilled, behold, there were twins in her womb." — Genesis 25:24 (ASV)

And when her days to be delivered were fulfilled The nine months were up from the time of her conception; or, as the Targum of Jonathan, when the two hundred and seventy days she went with child were completed.

behold, [there were] twins in her womb as was perceived by the midwife; a double mercy was granted, more given than asked for; probably only one child was asked for, but two given.