Комментарий к Берешит 38:27

בעת לדתה AT THE TIME OF HER BEARING — But in the case of Rebecca Scripture says (25:24) “And when her days to give birth were fulfilled” — in the latter case the months of pregnancy were complete, here, however, they were short of the full term (Genesis Rabbah 85:13).

והנה תאומים בבטנה, she recognised shortly before giving birth that she was going to give birth to twins. This is the reason the midwife tied the red string to the baby first appearing out of her womb.

ויהי בעת לדתה, it is clear from this wording that, in contrast to Rivkah, Tamar had not had any warning signs that she was carrying twins in her womb. There is an aggadic commentary according to which Rivkah’s problematic pregnancy was due to her carrying the seeds of two monarchs within her, whereas seeing that from Tamar only one potential king would emerge, the one being born first had “burst the boundaries,” excluding the other fetus from competition. (similar to Bereshit Rabbah 85,15.) Micah 2,13 alludes to this.