Sex assigned at birth: Biologic fact or farce?

Publish date: November 8, 2023

By K. Ashley Brandt, DO

Dr. Brandt is an ob.gyn. and fellowship-trained gender-affirming surgeon in West Reading, Pa.

Knowing the sex of a developing fetus is a common question many expectant parents ask at their prenatal appointments. While the sex of a fetus has minimal clinical significance to obstetrician/gynecologists, technology has made ascertaining the answer to this question much more accessible.

In addition to detecting certain genetic abnormalities, both noninvasive prenatal testing (NIPT) and preimplantation genetic testing (PGT) can discern the chromosomal sex of a fetus prior to birth. At the 20-week anatomy scan, the ultrasonographer can detect the presence of external genitalia to determine the sex. In fact, when a baby is first born, obstetrician/gynecologists are consistently asked “do I have a boy or a girl?” Assigning the sex of a newborn is one of the many tasks we complete in the delivery room. However, some of you reading this article would disagree.

“You cannot assign sex at birth.” “Sex is fixed, you cannot change biology.” These are examples of statements that frequent the comments section of my medical articles and plague professionals who treat gender diverse patients. I would argue, as would many biologists, scientists, and physicians, that these statements oversimplify biologic reality.

The term “sex” has multiple meanings: It can allude to the act of reproduction itself, but in the context of sexual determination and sexual differentiation, it can refer to the biologic and structural composition of a developing human. Within this paradigm, there exist three definitions for sex: chromosomal, gonadal, and phenotypic.

Chromosomal sex refers to the genetic makeup of a human, typically XX or XY chromosomes. There are also variations within this seemingly binary system. Embryos can have an extra sex chromosome, as seen in Klinefelter syndrome, which is characterized by XXY karyotype. Embryos can also be devoid of a sex chromosome, as observed in Turner’s syndrome, which is characterized by an XO karyotype. These variations can impact fertility and expression of secondary sexual characteristics as the type of sex chromosomes present results in primary sex determination, or the development of gonads.

Most often, individuals with a chromosomal makeup of XX are considered female and will subsequently develop ovaries that produce oocytes (eggs). Individuals with XY chromosomes are deemed male and will go on to develop testes, which are responsible for spermatogenesis (sperm production).

Gonadal sex is the presence of either testes or ovaries. The primary function of testes is to produce sperm for reproduction and to secrete testosterone, the primary male sex hormone. Similarly, ovaries produce oocytes and secrete estrogen as the primary female sex hormone. Gonads can be surgically removed either via orchiectomy (the removal of testes), or oophorectomy (the removal of ovaries) for a variety of reasons. There is no current medical technology that can replace the function of these structures, although patients can be placed on hormone replacement to counter the negative physiologic consequences resulting from their removal.

Secondary sex determination, or sexual differentiation, is the development of external genitalia and internal genital tracts because of the hormones produced from the gonads. At puberty, further differentiation occurs with the development of pubic and axillary hair and breast growth. This process determines phenotypic sex – the visible distinction between male and female.

When opponents of gender affirming care state that individuals cannot change sex, are they correct or false? The answer to this question is entirely dependent on which definition of sex they are using. Chromosomal? Gonadal? Phenotypic? It is an immutable fact that humans cannot change chromosomal sex. No one in the transgender community, either provider or patient, would dispute this. However, we can remove gonadal structures and alter phenotypic sex.

The goal of gender affirming hormone therapy and surgery isn’t to change chromosomal sex, but to alter one’s phenotypic sex so the physical body a patient sees, and others see, is reflective of how the patient feels. In fact, many cisgender individuals also revise their phenotypic sex when they undergo augmentation mammaplasty, penile enlargement, or vulvoplasty procedures for the exact same reason.

Circling back to the debate about whether we can “assign sex at birth,” it all depends on what definition of sex you are referencing. At birth, obstetrician/gynecologists most often look at the phenotypic sex and make assumptions about the genetic and gonadal sex based on the secondary sexual characteristics. So yes, we can, and we do assign sex at birth. However, in the case of intersex individuals, these physical characteristics may not align with their gonadal and chromosomal composition.

In the case of an infant that has a known XY karyotype prior to birth but a female phenotype at birth (as seen in a condition called complete androgen insensitivity syndrome), what sex should be assigned to that baby? Should the infant be raised male or female? A lot of unintended but significant harm has resulted from providers and parents trying to answer that very question. The mistreatment of intersex patients through forced and coercive medical and surgical treatments, often in infancy, should serve as a dark reminder that sex and gender are not as biologically binary as we would like to believe.


Moore KL and Persaud TVN. The urogenital system. In: Before we are born: essentials of embryology and birth defects. 7th ed. Philadelphia: Saunders Elsevier;2008:163-89.

Standring S. Development of the urogenital system. In: Gray’s Anatomy, 42nd ed. Philadelphia: Elsevier;2021:341-64.

Escobar O et al. Pediatric endocrinology. In: Zitelli BJ, ed. Zitelli and Davis’ atlas of pediatric physical diagnosis 8th edition. Philadelphia: Elsevier;2023:342-81.