When is a person’s sex determined? Trump has no conception, experts say.

On his first day back in power, President Donald Trump issued an executive order on gender that essentially states that males become males and females become female “at conception.”

But that, medical experts say, is wrong.

All fertilized eggs or embryos are filled with both male Y and female X chromosomes, and the gender doesn’t reveal itself until weeks later, they said. And even then, it’s sometimes hard to tell whether it’s a girl, a boy, or a baby with the sexual characteristics of both.

“We refer to conception as fertilization and nobody is male or female at fertilization,” said Dr. Eve Feinberg at the Northwestern University Feinberg School of Medicine. “Everybody has some combination of X and Y chromosomes, but it’s not until nine to 13 weeks that the differentiation of sex organs begins to develop, which is not always a binary ‘male’ or ‘female’ pathway.”

Titled “Defending Women from Gender Ideology Extremism and Restoring Biological Truth to the Federal Government,” Trump’s order purports to be a broadside against “ideologues who deny the biological reality of sex” and those who have “used legal and other socially coercive means to permit men to self-identify as women” and barge into everything from “women’s domestic abuse shelters to women’s workplace showers.”

The Trump administration, it states, will “defend women’s rights” by “using clear and accurate language and policies that recognize women are biologically female, and men are biologically male.”

It also proclaims that there are only “two sexes, male and female.”

“‘Female’ means a person belonging, at conception, to the sex that produces the large reproductive cell,” the order states. “‘Male’ means a person belonging, at conception, to the sex that produces the small reproductive cell.”

Not true, Dr. Aileen Gariepy, an obstetrician and gynecologist at Weill Cornell Medicine, wrote in an email.

“The new administration’s definitions of ‘female’ and ‘male’ are inaccurate, overly simplistic, and gloss over the complexities of defining what ‘sex’ is,” Gariepy added.

“‘Sex’ can refer to aspects of anatomy (such as internal organs or external genitalia), physiology (such as hormone milieu), or genetics (such as chromosomes),” Gariepy wrote. “And even these traits do not always match.”

Even though biological males have XY chromosomes and biological females have XX chromosomes, that’s not always reflected in their anatomy.

For example, the doctor wrote, a person with androgen insensitivity syndrome, a rare genetic condition, “may have XY chromosomes and be born with a clitoris and labia. And a person with XY chromosomes may not produce sperm (presumably the “small reproductive cell”) and similarly a person with XX chromosomes may not produce eggs.”

NBC News reached out to chief Trump administration spokesperson Karoline Leavitt to find out how the White House squares the executive order with the scientific explanation for when and how sex is determined in the earliest stage of pregnancy, and whether any doctors were involved in drafting the executive order. Leavitt did not immediately respond.

Dr. Jack Turban, pediatric psychiatrist and author of the book “Free to Be: Understanding Kids & Gender Identity,” said Trump’s “reproductive cell” (i.e. egg cells and sperm cells) definitions of male and female are among “the stranger definitions” he’s seen.

“Does the government plan on isolating people’s reproductive cells and measure them?” he added.

The truth is, Turban said, sex is complicated and can be defined in many ways: based on sex chromosomes, based on internal sex organs, based on external genitalia, based on reproductive cells and other factors.

“For most people, these sex domains align,” he said. “But for intersex people, they don’t. We’ve been watching politicians try to gloss over the reality that sex is complicated for years, and this is just another attempt to gloss over that complexity.”

Intersex is an umbrella term for those born with reproductive or sexual anatomy that doesn’t neatly fit the boxes of “female” or “male.” Androgen insensitivity syndrome falls under the intersex umbrella. 

Trump, during his campaign, relentlessly targeted the transgender community to rally his MAGA base.

In a report for the UCLA School of Law’s Williams Institute, authors Elana Redfield and Ishani Chokshi wrote that it’s no accident that Trump’s “gender ideology” order redefined “the word ‘sex’ in federal programs and services to refer only to biological characteristics ‘at conception’ and as unchangeable.”

“The definition explicitly excludes gender, gender identity, and any other characteristics,” they wrote. 

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