As mentioned in (“Genderally Speaking”), there seems to be a difference of opinion amongst Chazal regarding Leah’s giving birth to Dina. One opinion is that Dina and Yosef switched wombs. The other, that a male fetus inside of Leah turned into the female Dina.
What is interesting to note is that Chazal (פרד”א פל”ו, סדר עולם רבה פ”ב) say that 11 Shevatim (excluding Binyamin) and Dina were born within 7 years, beginning from Yaakov Avinu’s marriage to our foremothers. The commentaries have various ways to calculate when exactly the individual Shevatim were born. The Radal in his commentary on Pirkei D’Rebbe Eliezer (אות ל”ז) cites the Vilna Gaon who says that all of them were born after 7 month pregnancies, consecutively. This works out as follows: 7 months (of pregnancy per child) x 12 children (11 males and 1 female) = 84 months. 84 / 12 month in a year = 7 years.
There are other ways to learn, but according to this, let us go back to the first paragraph. According to those who learn that the male fetus inside of Leah changed into the Dina, everything seems fine. If, however, we learn that Yosef and Dina switched places, they would have had to both been growing at the same time, which seems not to fit the calculation of the Vilna Gaon, who learned that each of the children were conceive after the birth of the preceding one. Unless we are to understand that on the very day that one was born, the next was conceived. If so, it would emerge that on the day Dina was to be born from Rachel, Yosef was conceived by Leah. After the payers, Leah, who had just conceived with Yosef, gave birth on the very same day to Dina. Rachel, who had just endured 7 months of pregnancy with Dina, was to begin another 7 months in order to give birth to the newly conceived Yosef. Amazing. Does this not boggle the mind?