Embryo Change

How was Leah able to change the gender of her unborn child? Why did she do it? And what message is hidden in the Torah’s choice to conceal the reason behind naming Dinah, the daughter of Yaakov?

The Sages’s Discovery

“23andMe” is a genetic test that has become incredibly popular in recent years. With just a small saliva sample, it analyzes all twenty-three pairs of your chromosomes—the blueprint that reveals your ancestry and a surprising amount about who you are.

Here’s the basic idea: Every cell in the body contains twenty-three pairs of chromosomes—forty-six units in total—which store all of our DNA. Twenty-three of those come from the mother and carry her hereditary traits, like body structure, skin tone, and hair type. The other twenty-three come from the father, carrying his traits.

In twenty-two of the twenty-three chromosome pairs, the two chromosomes are identical. The only exception is the twenty-third pair, which determines the baby’s gender. The female chromosome is larger and called X, while the male chromosome is smaller and called Y. A female has two X chromosomes, and a male has one X and one Y.

The mother always contributes an X chromosome. The father can contribute either an X or a Y. If he contributes an X, it pairs with the mother’s X and the baby will be female. If he contributes a Y, it pairs with the mother’s X and the baby will be male.

But here’s where things get really fascinating: modern research has shown that even if the father contributes a Y chromosome, that alone doesn’t guarantee the embryo will develop as male.

It all comes down to an extraordinary system in how the male and female chromosomes interact. The small Y chromosome has to “override” the natural tendencies of the larger X chromosome. To do that, it must produce, very early in development, a special substance that pushes the embryo to ignore its built-in inclination toward developing as female and instead begin developing as male.

And here’s how delicate this process is: if the Y chromosome fails to produce that substance—or if something interferes with its effect—the female pathway wins by default, and the embryo develops as female, even if a Y chromosome is present.

In other words, a genetically male embryo (with XY chromosomes) can still, during pregnancy, shift and develop as a female—as if it had originally been made up of female chromosomes.

But until when is this possible? Until what day in the pregnancy is the gender of the child still fluid?

Using new scientific tools that let researchers observe what happens inside the tiny developing embryo, they discovered that this decision-making stage begins around day twenty-two of embryonic life and ends around day forty. After that point, the baby’s gender is fixed and cannot change naturally.

Now, here’s what’s amazing:

Our Sages, more than two thousand years ago—long before genetics, microscopes, or modern embryology—already knew about this phenomenon. They understood that an embryo could shift from male to female and even identified day forty as the point of no return. All of this, of course, without any scientific instruments. (Based on an article by Rabbi Zamir Cohen.)

Leah’s Children

In this week’s Torah portion, we read about Yaakov arriving in Charan. He meets Lavan and agrees to marry Rachel—but ends up being given Leah instead. Eventually he marries Rachel as well. Then the Torah says, “G-d saw that Leah was unloved, and He opened her womb” (Vayeitzei 29:31). She gave birth to a son and named him Reuven—meaning “seeing,” because “G-d has seen my suffering.” She then had a second son and named him Shimon—from “hearing,” “because G-d has heard that I am unloved.” Her third son she named Levi—from “attachment,” expressing her hope that now Yaakov would feel connected to her. Her fourth son she named Yehudah—from the word “thanks”: “This time I will thank G-d.”

But here’s the question: why does Leah say, “This time I will thank G-d”? Didn’t she have reason to thank G-d for her first three sons as well?

Rashi gives a powerful explanation. The matriarchs, he says, were prophetesses. They knew that twelve tribes would come from Yaakov and that he would have four wives. That meant that, on average, each woman would merit three sons. When Leah gave birth to her fourth, she realized she had received more than her “share”—and so she offered special thanks.

Now, Rachel saw that she had not borne children to Yaakov, and she decided to follow the example of her grandmother, Sarah. Sarah had given her maidservant, Hagar, to Avraham, and in that merit she was eventually blessed with Yitzchak. Rachel did the same—she gave her maidservant, Bilhah, to Yaakov as a wife, and Bilhah gave birth to two sons. When Leah saw that she had stopped giving birth, she did the same and gave her maidservant, Zilpah, to Yaakov—Zilpah also bore two sons. Afterwards, Leah resumed bearing children and had two more sons. At this point, Yaakov already had ten sons: six from Leah, two from Bilhah, and two from Zilpah.

Dinah

Then the Torah tells us something unique: “Afterwards, she gave birth to a daughter and named her Dinah.” The Sages noticed something unusual. For every one of the ten sons, the Torah explains the meaning behind the name. But when it comes to Dinah, we are told only the name—without any explanation.

Rashi quotes the Midrash that Leah took a hard look at her situation and said to herself: “If this baby is another boy, my sister Rachel will end up with fewer children than even the maidservants, who each have two.” So she prayed for the outcome to change—and the developing child became a girl. That’s why she called her Dinah, from the word din, meaning “judgment.”

But this leads to a major question. The Mishnah teaches that if a woman is already pregnant, praying that the baby be a boy is considered a meaningless prayer (Berachos 54a). If the child’s gender is already determined, such a request can’t change anything. So how could Leah pray for her unborn son to become a daughter? Isn’t that exactly the type of prayer the Mishnah says has no effect?

The Talmud answers: a person can pray about the gender of the embryo only until the fortieth day of pregnancy. After forty days, the baby’s gender is already fixed. And the Gemara says clearly that Leah’s prayer took place within those first forty days (Berachos 60a).

More than two thousand years ago, the Sages of the Mishnah stated something that modern science discovered only in recent decades: that until forty days, the gender of the embryo can still change under certain conditions. It’s not that our Sages were scientists—they were teaching and transmitting the Oral Torah with divine inspiration. Through that, they understood something that took scientists thousands of years to uncover.

A Lesson

The Rebbe raises an important question (Biurei HaChumash, Bereishis vol. 2, p. 75.): If Leah named Dinah out of deep sensitivity to Rachel—so Rachel wouldn’t end up with fewer children than even the maidservants—why doesn’t the Torah say so openly? Why is this reason left for the Midrash?

The Rebbe explains that with all the other tribes, the meaning behind their names was something everyone knew and felt. Their mothers expressed it openly, so the Torah records it openly. But Dinah was different. Leah never shared her reasoning with anyone. To reveal it would have caused Rachel embarrassment. Since the motivation existed only in Leah’s private heart—Leah “judging herself”—the Torah, too, keeps it private and does not spell it out.

And what does this teach us? That sometimes the greatest acts of kindness are the ones done quietly. When we put someone else’s dignity ahead of our own ego—when we help without announcing it, without saying, “Look what I did for you”—that’s real chesed. That’s “walking humbly.”

And when we act that way, the Rebbe tells us, G-d responds in kind. He blesses us in ways we don’t even realize, smoothing the path before we ever call out to Him—“Before they call, I will answer.”