The Short Answer
No, not all babies start out as boys. During early development, all human embryos initially develop with gonads capable of becoming either testes or ovaries, regardless of genetic sex. The embryonic gonads are bipotential and can differentiate into either male (XY) or female (XX) reproductive structures. Sex differentiation occurs later in development based on chromosomal instructions.
Sex Determination in Humans
In humans, sex is determined at conception by the sex chromosomes contributed from each parent. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The Y chromosome contains a gene called SRY that triggers male sexual differentiation.
During the first few weeks of embryonic development, the gonads of both male and female embryos contain supporting cell precursors and germ cells, but no differentiated structures. At around 6-7 weeks gestation, the SRY gene becomes activated in males and induces the differentiation of the bipotential gonad into testes. In the absence of SRY in females, the gonads follow the intrinsic pathway to develop as ovaries.
Therefore, all embryos develop identically at first, with the potential to become either male or female. The eventual developmental pathway is set in motion by the genetic sex established at conception.
Sex Differentiation
Once the gonads differentiate into either testes or ovaries, they begin to secrete different hormones that cause the development of the internal and external genitalia.
In males, the testes secrete testosterone which stimulates the development of the male internal reproductive structures (epididymis, vas deferens, seminal vesicles, prostate) and external genitalia (penis, scrotum).
In females, the absence of testosterone allows the internal reproductive structures (fallopian tubes, uterus, upper vagina) and external genitalia (clitoris, labia, lower vagina) to develop along the intrinsic female pathway.
Therefore, the initial appearance of the external genitalia is somewhat ambiguous or intermediate before sex differentiation occurs under the direction of hormones from the gonads. This gives the impression that all embryos start out as female by default, unless testosterone is present to stimulate virilization of male structures. However, genetically the sex is already determined at conception.
Abnormal Sex Differentiation
In rare cases, abnormalities in chromosomes, hormone signaling, or enzymes required for sex differentiation may lead to disorders of sex development (DSDs). This can result in atypical combinations of internal and external reproductive structures that don’t clearly fit typical definitions of male or female.
Some examples include:
- XY individuals with complete androgen insensitivity syndrome (CAIS) who develop outwardly female
- XX individuals with congenital adrenal hyperplasia who develop outwardly male
- Individuals with ambiguous genitalia or both ovarian and testicular tissues
These rare DSD cases illustrate that sex differentiation in humans does not always follow the typical male or female pathways. However, they do not mean that all embryos start out as female by default. The genetic sex is still determined at conception.
Conclusion
In summary, while early embryos have the capacity to develop either male or female reproductive structures, their genetic sex is already determined at fertilization by the sex chromosomes. Although the embryonic gonads and genitalia initially appear sexually indifferent, sex differentiation into male or female is directed by genetic instructions once the testes or ovaries form and begin secreting hormones. Disorders of sexual development can lead to exceptions, but these do not mean all embryos start out as females. The statement that “all babies start out as girls” is therefore inaccurate.
References
- Gilbert, Scott F. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Sex Determination. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10075/
- Ostrer, H. (2014). Disorders of sex development (DSDs): An update. Journal of Clinical Endocrinology and Metabolism, 99(5), 1503-1509. https://doi.org/10.1210/jc.2013-3690
- Biason-Lauber, A. (2010). Control of sex development. Best Practice & Research Clinical Endocrinology & Metabolism, 24(2), 163-186. https://doi.org/10.1016/j.beem.2009.11.002
Key Takeaways
- Genetic sex is determined at conception by the sex chromosomes (XX or XY).
- Initially, the gonads of all embryos have the capacity to become testes or ovaries.
- The Y chromosome gene SRY directs differentiation of the testes in males.
- Sex hormones from the gonads direct development of the internal and external genitalia.
- Abnormalities in sex differentiation can lead to disorders of sex development (DSDs).
- The statement “all babies start out as girls” is inaccurate.